Seriously, Dude. You don't have one, not one friend that is not completely insane? Someone who will say to you, "Uh, I hate to burst your bubble, Dawg, but you cannot sing."
You see people with ten or fifteen 'friends' waiting for them outside the audition room and you are thinking, how does one person accumulate that many completely tone deaf friends?
Again I sit here with my ears bleeding. Mind you, I watch the first episodes religiously simply for the terrible auditions. After that it gets boring, week after week. Unless we luck out and there is another Sanjaya Malakar in the mix that makes it onto votefortheworst.com and keeps surviving every week even though they couldn't carry a note in a bucket.
Of course you can tell when you are going to hear someone amazing because the show takes the time to interview the contestant at home. No surprises there.
You can also tell by the outfit a person is sporting if you are about to witness a train wreck. Seriously, if they have half their hair dyed (badly) red and the other blue, and can't put together a shirt to match their pants, you can pretty much tell it is going to be painful to watch and listen to.
And just an FYI. If you think you need a gimmick? You probably do.