First, you have to do the obvious visual inspection. Look for cracks in the housing/case, the shaft, or wear on the case around the shaft. Check for overall fit, looseness, etc.
Last, put some axles in it, or axle stubs that have been cut down (if you have some). Lock one end in a vice or have a friend try to work one axle while you work the other. That's about all you can try without disassembly. If something moves, it needs work.
Too many cars, too many parts, too little coin, too little space to work in, too little time left to make it all happen!
Update: down to one ride, still too many parts, a little more jingle in the pocket, gaining space, and it's going to happen this year!
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