There is no physical change. It's all psychological. If the guy shows confidence and is usually relaxed around women, he's probably experienced sexually. If you ask him if he's been out with a lot of girls and he doesn't have to stretch for an answer, just says "a few" or "here and there" right away, I would say he's probably experienced.
By the way, bringing up this question can be charming, but don't dwell on it. You then start to be obsessive.