How To Check A Differential Out Of A Junkyard Vehicle?

Lets say you're gonna remove the whole complete differential housing with axle tubes and all from a wrecked vehicle to install in a 91 Mustang GT.What kind of checks could you do to the housing before buying it?If you pulled the differential cover off,what do I need to look for??If I spin the wheels and don't hear any roaring noises,is that a sign that the bearings are in good shape??Give me the overall list of things I should check or be looking for.I already know that if a bunch of metal flakes or pieces of metal is in the gear oil or sitting on the bottom of the differential case,skip that differential and move on to the next one for inspection.Thanks for the help.BTW-the differential is being installed in a 91 Mustang GT with a 347 stroker.I'm replacing it because the previous owner done something to cause both axle tubes to be bent.I would rather install a used differential than try to repair an issue like this.The tubes are also twisted so I'm assuming this car was raced a lot.The previous owner had battle boxes welded to the car where the upper control arms attach to.This area normally cracks so I guess the battle boxes no longer get damaged due to their strength,but it has caused the axle tubes to take all the abuse now.