One sure sign of overtraining is having to force yourself to go to the gym. If you're dreading going to the gym, take a day off. If you've been working out real hard for months, and just feel exhausted, take a week off. Its OK to take a break now and then. There's more to life than the gym.
Sleep is important for getting big, too.
I do best on 8 hours of sleep a day. Some serious bodybuilders I know need 12 hours every day. Listen to your body. When it needs more sleep, make sure it gets it. You should be in and out of the gym within 90 minutes.