Get a Dream Theater CD and play it loud. If you still can't hear the bass, they're probably dead.

My guess is that you have not put the crossover capacitors in the door speakers so you can't hear the subs behind the seat.

Move one of the seats all the way forward and crank some tunes. Put your hand on the subs. If it's working you should be able to feel it.

If the moles start climbing out of holes in the grass and go running, they're probably working.