The feedback that I value most in determining what works and doesn't work is that of other musicians. Problem is that the participants in many song circles often don't want to hurt others' feelings and can be overly non-critical. So, you can't assume that a fairly warm reception is a genuine indicator of a well-done song. Even the guy who just bought a guitar last month is going to get some claps. It's when the response is plainly above the norm, or when someone compliments a song after the session is over, or when someone requests a song that they heard at a previous session that you know you've got a keeper.