I have bought and sold a few motors and the best way to tell if one is good in a junk yard situation is to pull the thermal reactor ( exhaust manifold for you piston guys ) and turn the engine slowly by hand to where you can see the apex seal in the exhaust port.
Take a plastic tipped object ( I use a windshield installation knife ) and check to see if the seal will push in. I have seen these stuck before and have loosened them with some WD40.
I also check the top of the rotor housings for any sign of oil leaking (bad O ring seal between housings)
I have had a 95% success rate using this method.

Rick Thompson