You can also check the square of the frame by locating two fixed points that are relatively a decent distance apart on one rail, and then finding the the same exact mirror locations on the other side. I usually used suspension bolts or the like because you know they will be in the same fixed location from one side of the car to the other. Then measure cross-ways from the front point on one rail to the back point on the other rail. Then repeat on the other side. If the measurements are the same for both, then everything is square, if not you have what we used to call a diamond in the frame. Usually this occurs when you have a heavy impact on one rail from the front, or the back.
If you have a buddy in the autobody field, you might be able to talk them into printing out the frame diagram, and usually it will give some specifics in measurements, and what the tolerances of those measurements can be within to maintain spec.
We also used simple hanging gauges that I would assume any body shop would still use for quick assessments (this was in the 90's) that could show you within reason whether your rails were level, and show through a vertical center pin on each gauge how centered your line down the middle was.
Hope this helps!