Here's a recent one with a good photo that shows roughly where the cable is located at beneath the car where it attaches to the transmission. Hopefully, yours doesn't look like this.
Speedometer cable pic
Here's a concept. Looking at the cable to see if it turns while driving... Brilliantly simple. Maybe worth a try.
Easy question about 300D speedometer
Warning, it appears there may be more than one style of cable. I didn't know that until just now.
speedometer and odometer inop
So, Maybe try out driving with the cable disconnected and seeing if it turns would be a good place to start. I've never tried it, so I can't say how difficult it is to watch the cable end and drive at the same time, but it seems like a decent, quick test.