Wow, this was the topic of one of my first-ever AH answers, specifically focused on late medieval England:

English alehouses, specifically, undergo a transition in the later 14th century. Brewing ale was a fairly standard in-home chore--ale was part of the daily diet. At the beginning of this time frame, ale houses were most often literally "houses that sold ale." A family with another baby on the way or compensating for the loss of a valuable livestock animal would brew extra ale one week, put up some kind of sign outside to notify fellow villagers or townspeople of ale for sale, and hope they reeled in customers looking for a drink in good company.

Towards the end of the century, alehouses start to become more permanent establishments alongside their wine-, food- and lodging-providing counterparts. I'm not personally familiar with a decisive argument for the shift, although the drop in grain prices around that time would have made mass-brewing of ale significantly more economically feasible and potentially profitable. In the earlier part of the century, ale was sold as often for consumption off premises as in the alehouse, so we can safely assume payment before drinking in those cases. Additionally, if you were simply popping into your neighbor's house for a drink and a chat (or a knock-down, drag-out barfight!) at the end of the day, it would be pretty hard to get out of paying one way or another.

With more permanent establishments--and here I'm going to broaden the scope to taverns and inns as well--court records can be ambiguous as to the order of service and payment, although it seems like payment came last. Ruth Mazo Karras has a couple examples of a "tavern guest" meeting up with an accused prostitute and her pimp:

"The ostler conveyed her to the George into a chamber there and after came the priest...they went to supper all three and the priest paid for all." (Common Women, 72)

Payment for lodging and food after the fact seems to be supported by literary sources--in the 12th century French Erec et Enide, Erec pays the duplicitous host on his way out the door; the conniving tavern host-priest (...always a priest) in the fabliau Le prestre et le chevalier lays out a lavish meal to the knight's delight--then proceeds to charge him for each item down to individual flecks of pepper.

Indeed, these literary sources paint a pretty grim picture of the public reputation of hosts-for-profit--and alehouse keepers are no exception. If anything, the situation is worse: they're a frequent character in artistic depictions of hell, and two of the seven personified Deadly Sins in the English poem Piers Plowman are associated with brewing ale.

Which is to say, take the source record on alehouses for what it is: people dealing with a vital, omnipresent, but morally ambiguous aspect of their world, with all the biases that entails.

That said, when ale-sellers show up in court records (at least, for reasons unrelated to licensing), it's generally because of customers' accusations that the sellers cheated them on measurement. This probably reflects a combination of alewives as an easy target with the alehouses' roots as a house: your "cup" or "glass" of ale was probably any old container the owner had on hand, so standard measurement was a judgment call. Still, though, the general paucity of reverse accusations--sellers suing customers for lack of payment--suggests that brewers were pretty good at preventing people from skipping out on their payment. (Also, if people skipped their tabs, presumably they wouldn't be suing over dishonest measurements.)

One of the most well-known satires of ale sellers, John Skelton's The Tunning of Elynour Rumming, portrays an alewife who squeezes every last bit of payment out of her market. She chases away people who can't pay

With a 'Hey, dog, hey! Have these hogs away!'

With 'There's a stench--gather up, you wench!

Take up dirt and all

And hurry out of the hall

collects payment in anything and everything that people can offer (even encouraging customers to go into debt):

And so comes another, dry as the other,

And with her she [the customer] brings meal, salt, other things

Her harvest girdle, her wedding ring

To pay for her scot as comes to her lot

and, um, enriches her ale with a secret ingredient:

And sometimes she blends the dung of her hens

And the ale together; and says, Everyone, come hither,

This ale shall be thicker and flow the more quicker.

Obviously, or at least one hopes, that is satire, and your good neighbor wouldn't sell you chickenshit in exchange for your wedding ring. Although Elynour mostly comes out on top in the poem (which is very English--in contemporary German literature, transgressive women never win), the poem suggests that the people she's kicking out for lack of payment have already consumed at least some of her ale:

With, Get me a staf--the swine ate my draft

Strike the hogs with a club,

They have drunk my swilling tub

Again--this is satire. But if we take the method of interpreting satire as rooted in reality so as to seem realistic though exaggerated, at least some of the time, alehouse guests by the end of the Middle Ages were drinking up before paying up. This would reflect the norms already established by taverns and inns, back in the days when alehouses were transient neighborhood affairs.

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There was an additional answer in the thread from another user, focused on the Holy Roman Empire, but they have since deleted it.