There is no clear warning of the end of act, though the main quests are usually obvious. In act 1 there are 2 ways to enter act 2, and you can complete any remaining quests before doing so.
Barta; you are correct. I am just before this point, so was being a bit conservative. I know you have to accept the witch's challenge to talk to the bishop to enter the catacombs, but wasn't sure where the point of no return was.
