When I was a CrossFitter, I could always tell because I would struggle to improve. If I went a few weeks without going faster/heavier/better, I was overtraining. I had all of the other normal symptoms at this point, especially poor sleep quality.
If you do more conventional lifting, then plateaus are not necessarily from overtraining. I always time myself between sets. Normally, It's to slow me down to the appropriate 2 minutes. When I'm burning out, I feel like the clock is pushing me when I'm not ready. I also just generally don't want to be there.