How to tell if a trans is functioning
 

I have pulled a trans from a parts car and am trying to find out if it is good. Basically, when I spin one side of the trans the other side doesn't move at all. It doesn't matter which side I spin the other side just sits. Am I right in thinking that it is "busted?"

Spin both at the same time in the same direction. Look at the input shaft and make sure it is turning also. Start out in 1st gear, and then check each gear including reverse. You can change the gears with it out of the car. Use a pair of locking pliers on the shifter shaft.

Drain the fluid and look for metal.. If you are going to use it, you should change the fluid anyway.

I think the other flange is always supposed to spin (the same direction if LSD, opposite if open diff) regardless of whether it's in gear. Someone correct me if I'm wrong.

I think you are right but I'm not absolutely sure. That's why I suggested looking for metal in the oil. If it isn't doing that, there should be metal chunks in there.

Open diff and in gear you spin one flange the other flange will rotate in the opposite direction and the input shaft will not rotate. You spin both flanges in the same direction and then you should see the input shaft also starting to spin. You spin the input shat and the flanges should spin in same direction. I am assembling back my gearbox soon and if I don't see this pattern when all assembled I won't bother to put it on my car.