Get the front wheel off the ground, and give it a good spin in the normal direction of rotation. You should see some indication on the speedo.
The speedo system is composed of three parts: the speedo itself, the cable, and the drive (on the wheel). Disconnect the cable from the back of the speedo, and spin the wheel - you should see the inner cable rotating. If not, either the cable or the speedo drive are broken. Find the bad part and replace it.
This is one area that needs attention on almost all older bikes. The system requires regular greasing of the drive and cable. Do not over-grease the cable, or the grease will migrate up into the speedo and ruin it. You also need to lubricate the speedo itself. Most of them have a little hole in the threads where the cable attaches. Put a drop or two of tri-flow or liquid grapite into the hole and allow it to penetrate down to the rotating part. It should rotate with little or no resistance.