I don’t think Stolen Valor will help with Marines, I thought they were SEAL advocates only, but their site may give you someplace to start.
That said, in my mind, this is a lose-lose situation. If he’s lying, and even if you can prove it, military service means less to some people than others. If you expose him, and he’s well-liked, you may find your peers view you as a busybody.
If you know without a doubt he’s faking, but he’s not wearing the uniform or using it to get benefits, your best bet may be to just file it away and relegate him to scum status in your dealings with him. If there’s even a shred of doubt, I’d advise you to be very cautious...Anything you say or do can backfire on you in a big way.
A fake Marine was just prosecuted here under it. He was so ballsy, he spoke at a Veteran’s ceremony last yearl before he got caught. Yes, in front of tons of people in full uniform.
Most of them are SEALs but they do all services.
http://www.stolenvalor.com/