You have come to full draw when your forearm is in line with the arrow. You won't be able to draw much beyond that point if you're drawing with your back muscles, because your elbow will be rotating back toward your spine, as if you were trying to close a door behind you with your elbow, you will feel your shoulder blade move toward your backbone, and there will quickly come a point when you can draw no further.
If you are drawing with your arm muscles rather than your back muscles, I suppose it is possible to continue to haul back on the arrow after your have come to full draw, and I don't suppose this would result in an accurate shot. Maybe this is what you mean by overdrawing.
Perhaps a symptom of overdrawing is that you don't feel the sensation of your shoulder blade moving toward your backbone and coming to the place where it can move no further.