if your sub is blown, it will sound like a plastic bag, or its possible to blow only one coil if its a DVC (dual voice coil), in which case it will still put out a decent amount of sound, but it will still sound like a plastic bag. easiest way to tell is turn it down low, and put your ear next to it. sound like shit? yeah, its probably blown. the coils inside the speaker can only flew so far, if it flexes too far it will tear it apart. thus, blowing your sub. it can happen with constant constant use over a ton of time, or it can happen with a single really LOUD sound. sometimes if you press play and its been turned up really loud and it suddenly BURSTS into music, youll blow your speakers.