The best way is to use an ODB2 scanner to "ask" the PCM what the car's speed is.
For the 1998 and older the output of the VSS sensor goes to BOTH the cluster AND the PCM. Therefore IF the PCM speed and the cluster do not work that points to a problem in the VSS senor itself.
IF the PCM does have a valid car speed that points to a problem in the cluster itself.
This first step will give focus to trouble shooting.