Figuring out a faker is really easy to do if you've served in any type of military (even if there's been no combat tour involved). My first red flag is WHENEVER someone mentions kills. When someone asks me about my service, my response is usually very vague, to the point of saying I served and hope they let me leave it at that. If they inquire further (as some do), I'll just say it's probably not appropriate to discuss and change the subject. Most people respect that.

Whenever I suspect someone faking a military career, I just let them talk and tell their story. It takes very little time for them to contradict their own stories, and I never even let them know I figured it out. I just lose all respect for them and have very little to do with them going forward.