If you have an auto parts store nearby call and see if they can bench test a 24V starter. A lot of them used to be able to, but I don't know if they still can. Most will do it for free if they can. Or better yet see if there is a starter rebuild shop, as they can not only test it, but can usually rebuild it for way less money than a replacement.
The following testing comes with a warning: These starters are heavy and have a lot of moving mass. You need to do something to secure them so they can't move around while testing. If they are not physically secured they can potentially cause injury to you or cause damage to things around you. A large bench vise is good, or a ratchet strap to something large and immobile will also work. I'm not responsible if you hurt yourself testing a starter that isn't secured.
This video is a quick bench test for a starter. As with the warning above, this is a smaller starter in the video so it doesn't move much
If the solenoid is good the starter gear should kick out. If both starter gear and starter motor are good it will kick out and spin the gear. If it kicks out but doesn't spin then continue to test.
Different than the video, for the next test if it fails the previous one hook the positive up first and then just touch the negative to the case.
If it didn't kick out and spin, you can hook to the other side of the solenoid with the positive jumper cable and see if the motor itself spins (it won't kick out though)
If it kicked out previously and doesn't spin then the motor is bad.
If it does spin then it's the solenoid that is bad even if it kicked out previously.
