There's quite a few websites out there dedicated to busting posers. One of the biggest is homeofheroes.com. They had a hand in the Stolen Valor act, which our illustrious SCOTUS shot down and violating Article I.
As for your question, RJC nailed the answer. The posers are usually pretty easy to bust, especially the Marine posers. FYI, there is no basic at Camp Pendleton.
Best one I've heard is from a Navy SF buddy. Guy goes into a bar in Coronado and starts bragging about being a SEAL (he's not). Bonehead move given the large number of SWCC guys around. Short story is that they showed him the exit. Easy way to tell a SF poser from real SF, the real deal won't tell you about it, 99% of the time.