I have a problem with identifying bad singers.?

I am very involved in music, I am generally known as a good singer, I'm a soprano and got into Baltimore School for The Arts for my singing, I've taken 3 years of voice lessons, have started to play piano, but I cannot tell if someone is a bad singer. I can tell good singers, I appreciate a good harmony more than anything, and Seth Macfarlane's singing makes me swoon, but I can't tell a bad singer from an okay singer. I'm not talking about those horrendous people on American Idol, in fact listening to bad singing like that is actually horribly painful for me. But I can't tell the difference between people who my friends say are bad, but I don't hear anything particularly wrong with yet I wouldn't want to listen to if I didn't have to, and people who have an average voice. Why is this, and how can I learn to overcome this?