Does it matter if they are who they say they are? If you already don't trust what they're saying now, what do you expect them to tell you that will magically cause you to believe what they're saying? Simply use the information that you currently have available to you to help you determine whether this person is feeding you a line or not.
I worked in Army Intel back in the early 90s, and even though I didn't do anything spectacular, there was plenty of stuff I couldn't (and still can't) tell my friends and family. But what proof could I offer you that I actually did what I did? Other than showing off a couple of certificates or awards, there's not really anything I could do to prove anything to you, is there? After all, if I were to reveal classified information to you, how would you know I was telling you the truth?