How do I know if Rear is still good?

so im rebuilding my 68 dodge dart, and i have already stripped most of the car and pulled the tranny and engine, which will be receiving rebuilds.

Im wondering... how do I know if the rear end is still good? the car wasnt driving when I bought it, it was sitting in this guys yard and it wasnt registered in ten years. the funny thing is, theres still gas in the tank, cause i can hear it.

Is there anything I can do to find out? instead of rebuilding it, when i didnt have too?