In my opinion, there really isn't much to see with a "bench inspection", and that activity is pretty much worthless - not completely worthless, but not something that I'd consider a complete analysis.

The critical components that you should be concerned with are inaccessible without taking the gearbox apart. Generally, anything that turns.

I do not "take on faith" that any used gearbox is good. For the cost of a gasket set, I think it's worth it to disassemble and inspect properly.