Many times the ones I suspect the strongest have early signup dates. I'm paranoid enough to suspect that they've gotten access to an abandoned persona or somehow managed to changed the signup date.
It's nothing that can be proven. So answer the challenge when you meet it, if you're on the level.
I got in a little spat recently with a newbie troll who claimed to have been an FR member since 1995, but his sign-on date was only a few months ago.
He claimed he could prove it, but went quiet when I challenged him to do so.
But if he could, it would be interesting, as that would mean he had signed up before JimRob did.