checking cab for straightness

[stillwater]

How is the best way to check my pickup cab for straightness?

My Dad handed this truck down to me, he always thought it wasn't quite right.
Thinks cab/frame is bent. I've asked earlier about checking the frame, but how is a good way to check the cab?

And then, how to straighten it...I was thinking that with the roof, doors and windshield/rear window removed, it could be straightened with a Porta-Power jack. It seems that with this stuff all removed, it could be put into shape reasonably easy.

Problem is, when Dad purchased it some 30 years ago, it had been rolled down into a holler. And retrieved by that owner, who had it sitting in his garage with no time to work on it. It has been restored after we got it, but he never thought it was quite right.

I'm guessing I need the right measuring points, then see if doors fit properly...? Not sure