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(Notes: I'm going to focus on the United States for this answer, and on the first half of the 20th century; throughout I'm going to use the language of "gay men and women", "queer networking", and "gay subcultures" but a wide variety of language and self-descriptors were used during those five decades and these subcultures encompassed a variety of identities, including ones that we'd now call transgender. Life is a rich tapestry, etc.)

You’re definitely right about word of mouth — there were few ways to advertise gay meeting places explicitly as such without inviting unwelcome attention down on your establishment and clientele. But the good news is, meeting other individual gay locals might be your entryway into the local gay scene -- whether this was a network of locations and establishments in a major city like New York or Chicago, or a tip-off about a particular hole-in-the-wall bar in a neighboring town where a handful of like-minded people sometimes congregate. Rather than asking around everyone you know, you could narrow your inquiry to only asking individuals who seemed to already be in the know, or take a gamble on an encounter with a stranger and hope to glean useful information regarding places to meet other gay people in your own city and outside of it. This could include not only commercial establishments and cruising spots, but also the identities of individual gay men and women in other cities who in turn could be counted on as a contact in the local gay network.

One way to get the lay of the land in a new city was to reach out cautiously to receptive-looking strangers -- especially in public locations like bars and train stations, spots that might be used for hooking up with a sex partner on short notice and whose use for such purposes could become well-known even among heterosexuals and the police. Some types of rendezvous spots reliably appeared in a variety of major cities -- public parks, for instance, or public washrooms, or areas where sailors were known to congregate. Others might be more idiosyncratic to a certain location and only known to gay locals. Cruising or being cruised might not be the start of a lasting partnership, but many individuals seem to have kept in touch with their hookups and to have used such connections as the groundwork for further networking. Simply to allow oneself to be picked up could be the start of a lifelong involvement in the gay community -- either through a mentorship relationship with a more experienced, in-touch gay individual or by adding another clue to a stockpile of potential clues to where gay people might meet. Some of these meetings might even be facilitated by heterosexual gossip and hearsay -- the same piece of information, that such-and-such a place was frequented by fairies for instance, might mean one negative thing to a straight speaker while serving as a handy piece of intel for a gay listener's developing worldview. As the century progressed, references in literature and print media to homosexual practices and meeting-spots diversified, and even remarks meant as homophobic deterrents might provide plausible leads for a gay person interested in meeting or even just seeing other gay people, perhaps even for the first time in their lives.

One resort might be euphemisms and coded talk -- the latter might incorporate a slang vocabulary that was common to gay men and women, like the word "gay" itself. An individual might cagily allude to a gay topic or location and watch for any sign of recognition or interest. Thomas Painter, in his unpublished 1941 manuscript "The Homosexual", wrote:

Supposing one met a stranger on a train from Boston to New York and wanted to find out whether he was "wise" or even homosexual. One might ask: "Are there any gay spots in Boston?" And by slight accent put on the word "gay" the stranger, if wise, would understand that homosexual resorts were meant. The uninitiated stranger would never suspect, inasmuch as "gay" is also a perfectly normal and natural word to apply to places where one has a good time [...] The continued use of such double entendre terms will make it obvious to the initiated that he is speaking with another person acquainted with the homosexual argot.

This presupposes that both parties are familiar with gay slang and double entendres, and that both are sensitive to the shifts of intonation that indicate these terms are being used in their homosexual sense -- so it might not be accessible to, say, a man who's attracted to men but who only knows that men who have sex with men are called perverts and sodomists, not that they call their meeting-places and even themselves "gay". So that might not be accessible for a gay man operating on the Being Gay 101 level, but if it's successful, the speaker may have made a new friend, met a potential lover, and found out the names of several Boston gay bars all in one chance meeting.The use of euphemisms and oblique references to the topic however strikes me as more of an organic development under the constraints of societal taboos -- if you can't frankly discuss your own homosexuality and inquire about another person's eligibility without the danger of serious repercussions, or if you don't have an extensive vocabulary for your own identity and desires, it makes sense to talk around the issue and in that way to subtly gauge another person's interest and tolerance. Someone might notice that their new coworker is unmarried and ask cautious questions about whether he dates, for instance, or what he likes to do for fun, while inwardly gauging the risks and rewards of inquiring further. Even if such questioning struck the listener as odd, it could err on the side of not being actionable or actively arousing a specific suspicion of homosexuality.

An extension of coded language and camping was flagging via nonverbal gesture -- catching an individual's attention with inquiring eye contact, or another form of superficially innocuous social interaction, and using that interaction to gauge further receptiveness. This was easier to pull off in urban environments, where a certain amount of encounters with strangers was unremarkable -- it could be as unmistakable as beckoning someone closer or as subtle as the way one handled a pack of cigarettes when sitting at the counter of a diner, without even making eye contact. The end goal was to facilitate an encounter which could pan out into something useful for both parties, while to an outside heterosexual observer it might appear entirely innocuous.

Simply spotting other gay people out and about, or being spotted in the same way, was a viable, if risky, way of engaging with the local gay subculture. This could endanger status among heterosexual acquaintances, or result in violence and legal trouble, but people seem to have done it anyway, both on special occasions and on a regular basis. In the first half of the 20th century, highly visible gay individuals (including crossdressers) as well as more discreet gay men and women congregated in public places like restaurants to meet and mingle. Such establishments could gain a negative reputation for being the haunts of disreputable, criminal elements. This same visibility that put lower-class queer loungers in danger of police interference and social censure made it easier for like-minded individuals to find one another. Gay men and lesbians weren't the only people to use cafeterias and automats as social centers; these establishments were more accessible hangouts for working-class city-dwellers and artists than more exclusive public places with higher price points -- but many establishments were open late into the evening and this facilitated the kind of "double life" effect that in some ways characterized the gay experience in the 1930s through 1950s -- work all day in the straight world surrounded by your heterosexual friends and neighbors, and go out with your gay friends at night, with the assurance that the paths are unlikely to cross. Simply spending time in a cafeteria wasn't incriminating, and the working-class public nature of these spaces made it possible for straight onlookers to hold the spaces themselves as more or less innocent while still pausing to gawk at men in makeup and women in trousers. Accounts from this era testify to a lot of code-switching going on -- individuals switching between the language and speech patterns of respectable straight society and the slang and campy intonations used by those in the know regarding gay subcultures to indicate a shift of social mode from the mainstream to the queer. In George Chauncey's Gay New York, he quotes one Greenwich Village bohemian, the gay author and artist Parker Tyler, describing a visit to a Childs chain cafeteria in 1929:

"Well my dear considering that I was in a huge fur coat of Clairmont's [a female friend] and must have looked very gorgeous, it isn't a surprise but that waiter started right in camping just as though there were no law!! [italics his] And everybody in our party started camping after the waiter asked me: 'What will you have, gorgeous?', and I replied bitterly: 'Nothing you've got, dearie; which really did upset everyone. And you can imagine how things went from bad to worse. So I concluded Brooklyn is wide open and N.Y. should be notified of its existence."

(If a city was large enough to have neighborhoods associated with bohemian types and artists, those neighborhoods often doubled as early gay enclaves -- for Tyler and his New York contemporaries, these neighborhoods were Greenwich Village and Harlem. These associations with gay nightlife weren't well-kept secrets, and the promise of seeing gay men and women at play drew the interest of curious straight spectators as much as it attracted censure and hostility from condemnatory ones.)