The simplest thing would be if the cable failed. Open up the dash and remove the speedometer. Take the truck for a drive and see if the end of the cable is still turning (you probably should bring someone else along to do this!).
You can also test the gauge. Find something that will fit into a power drill that you can wedge into the gauge where the cable used to go. It should turn easily. If you do get it turning, you should be able to see the needle move.
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