Pain. I assume you are talking about physical training. If so, discomfort/pain in the areas subject to excess will be the cue that you are doing too much. Also, tiredness is another sign of excess. If you are talking about other training, let us know, so we can answer more specifically.
Pain. Simple logical answer isn't it? The key is localized pain. If you have general muscular pain that resolved after couple of days of rest, you are probably good. If you have however localized pain that keeps returning or not resolving, you should see your doctor and/or take a longer rest.