How to Check Uni body for Straightness

Bodyperson said
You could rig up some kind of scales with straight bars between them set to the same height from side to side and hang them off of the rocker panels, front and rear, to check for twist or barrow some centering gauges and learn how to use them.
If the center section is square and the sheet-metal fits, your good to go.
Now all you have to worry about is wheel alignment.

Thanks for all of the help Bodyperson. Didn't understand the significance of getting the center section square etc. Checking for a diamond condition seems pretty straight forward, kind of like taking corner to corner measurements when framing a wall.

If I understand what you are suggesting for twist, I would measure a set distance down from the rockers on both side of the car. Then run a straight bar from side to side in the front at that distance and then do the same in the back. After that I would run another straight bar between the front and rear bars and see if both sides of the car are level to each other?

This sounds the same as what the Great Ronzini was saying above.