Best way is to be familiar with ranks, medals, ribbons, etc. Stolen valor people always go overboard on awards and lots of them conflict (ribbons and medals from different services) and almost always have some sort of special operations pin or multiple.

If they are out of uniform, then it's crazy stories that are over the top and sound like a movie. I've known several guys who were in special operations and they don't talk about missions. In the rare times they do, it's very brief and usually about something non combat related or the combat isn't the point. "We were catching fire on a mountain and a fucking goat just walked up to us like nothing was going on."

Most combat stories are like this from people who have seen real combat. My father, who was 57 when I was born, was a tailgunner on TBF Avenger torpedo plane in WWII in the South Pacific. I was in love with WWII documentaries growing up and fascinated by WWII. Even still he only told me one combat story ever before he died. "We were doing a strafing run on a Japanese destroyer and the stern of the ship was just full of potted plants." And that's it, no other details.