checking rear end housing straightness

If your reared is complete and out it is very easy. Put tires on it and run a tape measure across the tread ( like checking toe ) rotate the housing 1 complete turn and watch your tape. Best if you have a helper. You may have to put the tires on top of blocks for clearance of pinion, brake lines and so forth. This is how we did all our rearends, even welding on the housing can warp it.