About to buy one, but old and easy to fix. And cash lol.

Yep, we use laser trackers at work to define flatness. I’m sure that much isn’t required, but it’s down to .003in, so pretty flat and it’s fast. But that’s a literal laser instrument designed to do such things and costs 250k. I guess you could with a faro arm or romer arm, but again, those are still 10s of thousands of dollars. CMM? Those can get reeeaaallly pricy