You can remove your speedometer and bench check it with an electric drill. Guess you could remove the cable from the transmission and attach a drill to the inner cable under the car and see if the speedo responds. If not put on some gloves and pull out that inner drive cable, may find that it is broken.
If you determine that the speedo spins OK then it is in the lower drive unit and the gear may be shot. If that is the case be sure that you order the correct one, there are a few different gear choices. Chap