If you want to find out if someone is legit, just ask them who gave them their black belt. You should be able to trace his instructor, his instructor's instructor, etc. back to the Gracie family. There is an exception to this, such as Ronaldo "Jacare" Souza whose lineage isn't necessarily from the Gracie family but I doubt you'll come across anyone like that, especially in the U.S.A.
It is b.s. there is no official registry. I'm sure Carlos Gracie Jr. and the IBJJF could easily do it but not without charging everyone an arm and leg to do so (like they do with everything else). Then again, can you imagine if you pissed off whatever organization for whatever reason what would happen? Joe Moreira got his black belt from Reylson Gracie but they've had a falling out. Reylson doesn't like to recognize Joe as one of his black belts anymore.
I always liked the idea of BJJ academies posting a list of who they promote at their schools on their websites, especially to black belt. This would make it a lot easier for people to weed out the frauds. Don't know why they don't.