iardon:

In my particular case, the telltale sign was smoke coming from my tweeter followed by no sound! (I'm not BS-ing you). The woofer damage was more subtle, but created a buzzing sound on certain bass frequencies. When I removed the woofer, I also noticed the cone would grate in places on the spider when you pushed on the driver. I think it must have melted part of the voice coil or something.

For less dramatic damage, listen closely to the tweeter(make sure the volume isn't too loud so you don't damage your hearing) and listen for any crackle or frying egg sounds. It's likely any damage caused by the feedback problem will take out the tweeter or midrange (if you have one) first before damaging the woofer. If you do try the push test (feeling for grating) on the woofer, make sure the music and/or power is off -- you don't want to push on a cone during playback. Also just push in on the driver carefully and shallowly, if the voice coil is in good shape, you should feel a smooth excursion on the driver as it moves back and forth.