I wear a watch.

(Sort-of-ex-)T and I didn't really have a fixed time length, as in, if I came in at 1 and she had another client, or had to leave, at 2, I'd obviously wouldn't stay until 2:30. But if she didn't have a client at 2, we might talk until 2:30. Often I'd ask at the beginning of the session how long we had; other times I'd ask shortly before 2 whether we had to stop at 2.
I'd keep an eye on the time, sometimes saying stuff like "But time's almost up" or "Don't you have another appointment?" but I wait to leave before she sends me out.

Pdoc is more flexible - we usually talk as long as necessary. (Phone check-ins are usually 10-20 minutes, but I think he schedules half an hour. In-person sessions are scheduled for an hour, but we're often finished around 45 minutes and sometimes we talk for an hour and a half) He rarely mentions the time, sometimes just gently 'works me out the door' (although he is never hurried; I'm not a doorknob confession kind of gal but I think if I were to make one, we'd talk about it for a few minutes even if he was in a hurry. Although we might also talk about it for a minute and then he'd say "But I think this can wait until next session" or "Shall we talk about this some more next session?") and once he mentioned at the beginning of our session that he really had to stop at x because an interpreter was accompanying his next patient. (I think that that time, he mentioned it so clearly because he'd also started the session late - but I might not be remembering that correctly)
Also, my psychiatry/therapy organization has multiple locations, and pdoc is in one and T and family T in another. When we had regular meetings where everyone got together (me, pdoc, T, family T, parents) pdoc would drive over to T and family T's location. It's about a 45 minute drive from one location to the other. Pdoc would often say "I have to leave at x" at the beginning of the appointment and then we'd go over anyway and 15 minutes later he'd say "but I really have to leave now". (I assume he schedules an hour for the drive back, and knows he'll only take 45 minutes, so if traffic isn't a murder he can afford to stay 15 minutes over time)