How old are you, what is your experience level; are you still making gains? What are your goals?
Whenever I read a Q and A, from a highly experienced pro body builder, they always say that the biggest mistake they made earlier in their careers, was over training. Having said that, my background comes from college athletics, where we were pushed hard: squatting, and dead lifting, benching, pull up, row. +accessory lifts -- 4sets, every other day. and on off days working our ass off, running, plyo's stairs, jumps etc. We were pushed so hard, several players were loosing their breakfast each session. Not to mention this was just the morning, we still had to practice in the after noon. So from my experience, it's hard to think that you're over training.
Now that I'm older and out of athletics, I look to the speed of the muscle contraction to determine if the muscle is ready to be trained; hard, or light, or not at all. If everything feels good and my muscle contracts quickly, then bring on the weight. However, if it feels slow and slightly unresponsive, than I will train it lighter, If it feels considerably slower, than I'll take a day off.
Moral of the story, listen to your body and look for the signs.