Providing you're relatively polite about it, people shouldn't get offended when you tell them they're not as good a singer as they think they are, amirite?

This applies to any kind of criticism.

Especially whn they wanna go on American Idol.

There's this one girl I know that thinks she's really good at singing. She always asks me and my friend (who happens to be and AMAZING singer) if we think she's good. There's really no polite way to say that she makes my ears bleed, so I just say she's good and my friend gives her some advice. I'd feel really bad if I told her I thought she sucks though :/