How do singers know when they really can sing a tune? I have watched Swedish Idol and some people really can't sing (maybe training would help). If I wanted to check if I could play Silent night on piano the simple way would be playing it and check if it seems allright or if I forgot something. This wouldn't work for vocals as you don't always hear if you hit the right notes. How do singers check if they really know a tune or just think they know (like those on Idol).