Protection circuitry generally just shuts the amp off. So that isn't it.
The common symptoms of a blown subwoofer driver:
-Unusually weak output (aka sensitivity goes to crap)
-Rattling sound (almost as if the driver is bottoming out at next to nothing)
-No output at all
-Cone or surround are torn or ripped away from the basket or each other (If you can pull the driver, inspect the spider for the same kinds of damage)
I may have missed a few. A damaged (burnt or maybe scuffed) voice coil is a form of "blowing" the driver, I believe (anyone want to chime in on that one?)