For me, calf and forearm pain are about the only indicators that I'm not recovering well. They're areas that never get sore normally, and whenever I start experiencing bouts of it, it won't be long before I hit some major stalls.
In terms of knowing when to stop BEFORE that happens, generally, if I don't want to train, I don't. Don't get me wrong, I don't enjoy training, but I enjoy getting stronger and I know training is necessary to accomplish that. However, if I find myself dreading a training session, I just flat out don't do it, as it's a sign that something is going wrong.
As for "what are signs you could be pushing yourself harder", I just measure results. If I'm making progress, I keep doing what I'm doing, because it's working. If I'm not making progress, I'm more inclined to add more than to take away.