It depends on the period of time in which you're in the press gang, how badly your captain needs men, and how much he might be inclined to stretch the rules to take them on board. To put it briefly, while impressment was meant only to apply to seafarers or men who had been aboard ship, the enormous manning requirements of the Royal Navy during times of war might result in anyone being swept up in a "hot press" without recourse.

I've written about this before, here and also here. I'm going to quote a bit from those answers below:

It very much depended on the time and place from which they were impressed. In theory, the impress service or impressment was supposed to take men who had "had use of the Sea," which would have limited it to people who had been aboard before. In practice, the impressment of each war or crisis worked out somewhat differently:

• in the Dutch wars of the 1650s-1680s, men would be pressed for a season, and could generally be counted to show up when they were meant to and to serve for the year aboard a ship. (The "press" or "impress" they received was actually an advance on pay that they would receive upon promising to show up for a ship.) As the Dutch wars continued and the Navy got larger, impressing just a portion of the men in the merchant or fishing services became less likely to work, and men started to resist the press, either actively (by running/not showing up once pressed) or passively (by avoiding press gangs).

• In the press that existed in the Seven Years war, pressed men started to be used aboard ships for longer periods of time (because the Navy started keeping ships in active service for longer). That led to more unrest for the men, who particularly disliked being turned over from one ship into another (in general, men were paid at the end of each deployment, with deductions for slops [clothes] and medicine they used during the deployment). When they were turned into another ship, they would be paid off for service in that ship only, and would receive a "ticket" for their other service which had to be cashed in London. In practice, most sailors were unable to go to London to cash it (and ran the risk of being pressed again), so they wound up selling their tickets at a discount.

• By the time of the French Revolutionary and Napoleonic wars, impressment was a major way to man the Navy. Popular and successful captains generally were able to man their ships with volunteers, while others had to rely on the press gangs. Those men grew less seamanly as the sale of the press increased and the manpower needs of the Navy increased, and the pressed men compounded the problems (or vice versa) faced by unpopular captains.

And from the second, in which I was answering some specific questions so the formatting hopefully makes sense:

In particular, the crews of whaling ships were supposedly immune, whereas the crews of Indiamen were fair prey. Was there any rationale for these immunities? Or were they a bunch of ad-hoc rules slapped together over the years?

Well, both to an extent. The whaling trade was important to Britain both as a source of revenue and of actual products (before petroleum was discovered, oil came from whales, after all, to say nothing of whalebone and ambergris). So an exemption for the whaling fleet or the coal fleet or some other group of people made sense. That said, the exemptions and the non-exemptions were certainly patched together over time -- London watermen were pressed at one point, despite having little use of the sea, for example.

The novels also talk about how English civilians could be snapped up by press gangs on shore; The Ionian Mission had a middle-aged botanist pressed while visiting a port city, and liable because he had "used the sea" in his youth.

In general, during the time of the A-M novels, "use of the sea" was a reason why a man might be impressed. If I remember that book correctly, he had an anchor tattoo or something like that? which showed he'd been to sea, and was a good enough justification for a captain looking to man his ship.

But how did press gangs figure out who was and wasn't a sailor who could be pressed? Could they just grab random people off the street for looking like sailors, and then sort it out afterwards?

In general, sailors tended to dress and groom themselves in particular ways not common to the civilian population (pigtails, tattoos, etc.) But certainly during a hot press, or on an overseas station, the press gangs might snap men up now and ask questions later.

Hopefully those hang together and make sense. Let me know if you have follow up questions.