I have little musical aptitude, but I certainly know that my singing absolutely sucks. I hear the song, try to repeat it, and go "wow, dude, I suck." So I think I'm good on the tonedeaf thing.
It's also what shocks me when I watch people suck it up on American Idol. I mean, I'm terrible, but i KNOW I'm terrible. These people genuinely seem to have no clue how bad they are. WTF?