Welcome to the fourms. You can test the starter clutch by turning the gear. It should turn freely one way and not turn the other way.
Also, I would take the starter off and use a set of jumper cables hooked to a known good battery to bench test it. Just hook one end to the battery and on the other end, hook the ground to the starter housing and touch the hot to the post on the starter. Make sure you hold on to the starter good, because if it's good, it'll jump out of your hand if not holding it tight.
My guess is the starter clutch is trying to lock up.