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