A general attitude change is a huge sign ... when people get into deep thought about their own life or a relationship they no longer want to be part of, their individual attitude becomes lots more volatile - they are upset easier, and they begin to not care about the other person any longer.
Other signs: He doesn't call you anymore, He avoids all personal contact, he goes the other way if encountered in a public place, never returns your calls, never replies to emails, etc etc.