if u have a meter u can measure the coils resistance.. a dual 4ohm voice coil, should measure close to 4ohms on each coil..
simple test, is to use a AA battery, and tap the speaker wires to it, u should hear a thump..
u can push the cone of the sub, in and out.. make sure it moves freely, if it rubs or makes noises, scrapes.. its blown!! If its froze, and cant move, its also blown!!
Actually, you sometimes can't push the cone by hand perfectly even enough to not scrape the voice coil at least a little. Also, some subs have very stiff suspensions on them.