Free tip for all you singers out there (especially the ones at noisy bars)--> Throw an earplug in one of your ears. It helps you monitor how healthily you're singing. I've been doing it for months now, and this being my first week performing with a cold, I gotta say it's a godsend. I can feel where the threshold of "destructive pushing" is, and I simply don't cross it :)
Bonus fact--> Most people have one ear that is far better at determining intonation. It shouldn't take much experimentation to determine which ear to plug and which to leave open.