As someone who is occasionally guilty of writing songs I try to remember Ernest Hemingway's wise words: "The most essential gift for a good writer is a built-in, shock-proof, shit detector."
And if you can't trust your own radar then watch the audience. Are they singing along or falling asleep?
I once asked Sid Kipper why he had dropped certain songs. "They weren't getting the laughs," he said.
By all means write songs but we should all try to be our own sternest critics - although that's hard given the levels of criticism you can get from some Mudcatters :-)