if hes in the military he has a military id and dog tags. they might be hidden but hes has to have them with him when he goes on base or into war or wherever. as far as the married part goes unless he got rid of it he also has a ring somewhere. if you want to truly know, you have to snoop. wallet, something he always keeps with him, facebook if possible, he might have two though. one for you and one for the wife, but sometimes guys are idiots so he might have just one.
If he's always away at weekends and national holidays, if he's got a circle of pale skin on his ring finger, if you only have his mobile phone number and not his landline number, and if you arrive at his house to find a woman with ten kids running around, I'd be suspicious.