Hopefully, you will eventually be able to tell the maximum amount you can train w/o feeling like shit. I assume you have been overtrained before (most people have), so don't you have some idea?
When I am overtrained, I get fatigued very easily. I get that post-squat want-to-puke-my-brains-out feeling too often. I try not to ever get to that point, because it is counterproductive.