There is a moment when the painting is finished and I recognize it. I'm not sure I can put it into words - but it's a recognizable feeling. Like finishing a good meal. There is a sense of satisfaction. In order to catch the moment though, you have to be watching for it, looking for it.
Too often I see students painting for the entire three hours of a class. Layer after layer of paint, putting it on, scraping it off, putting more on. I'm sure that many good paintings get painted over in this fashion. Because after all, as painters we like to paint! I love moving the paint around, getting excited about getting just the right color, the right shape. But, unless I'm very careful and watch for it, I will miss the completion point of the painting and paint right over it.
So, my recommendation (to myself and anyone else who cares to listen) is paint less and look more. Spend at least as much time looking as you do painting, and probably more. More time looking, less painting. Seems strange for a painter to look more than paint, but really, if we don't slow down, we could just miss the end of the painting.