I don't think there is a way to tell, really. I think it differs from dog to dog. I know several people who never noticed a difference in their males. With Cody, once he was used as a stud, he changed alot. He would hike his leg, not eat and sit and shake all over when there was a female in heat. Even when there wasn't, he'd just sit at the door and whine and shake sometimes.
He has since been neutered and no longer does these things, but it was enough to drive you nuts sometimes.
