I've read that with the housing completely empty, you take a regular lamp and stick the bulb up against one end of the axle housing, then look down the other. All the 'circles' should be concentric. I read that trick on pbb, and someone even had a picture posted in the thread.
An alignment bar is the only real way. Bolt it in where the carrier bearings go and see how far your housing is off. If you can visibly see it's bent by looking at the housing then it's pretty bad.