Buying a used tranny is a guessing game. Yeah you can try to shift through gears as stated before, BUT that don't mean it wont grind or pop out etc etc.

As far as the motor is concerned their is a few things you can do as custom240sx said, but as far as the other guy saying turn it over with an impact that just makes me smile. When you rotate the motor it is counter clock wise and it can loosen the bolt, if the impact is strong enough, BUT an impact by design will HAMMER once it see's resistance. When it hammers it will not rotate the motor and IF you get a drill strong enough to rotate the motor fast enough I would not want to be the one holding it because once it rotates and hits the compression stroke it will break your wrist.
Do what custom240sx said and also when oil pan is off also grab each rod and check for play or just pull all the main caps and rod caps and look at bearings.