Hmm. I don't go to many parties...but if going to a pub to have a few beers and listen to a band can be considered one...
Then I typically know it's time to leave when some insecure yahoo male jerk and his 3 friends are trying to provoke me to attack them by pretending to be gay, entering my personal space and making not-so-discreet suggestions. Target the tall guy for a pounding. Yep, it's time to go.