So what you're saying is you don't know how flat your plate is...
Lol, yes absolutely right!
Point is, when you buy a new surface plate or mill vise or any number of precision items from China, you also don’t know how flat your purchase is. So we are kinda stuck with this Faustian choice of used, once good, or new, possibly good. This is the dilemma of hobby or small time machinists everywhere.
There are things you can do to check a surface plate with tools you have. But without an autocollimator, you won’t know EXACTLY how flat your surface plate is. You CAN detect severe wear with a tenths indicator at the end of a surface gage mast, sweeping in from the corners. Suburban Tool among others has a good video on this. Maybe Robin Renzetti too.
This may help explain why I’m down this rabbit hole: a lot of Chinese precision stuff begins life as a steel casting. Castings move over time. Machining a casting can relieve internal stresses that causes the casting to move. US precision toolmakers had tricks. Some let their castings sit for a year etc etc
The internet is rife with stories of people who bought twisted mill vises, or warped table saws. My mini mill had a bowed column. If you have a surface plate, wherever you got it, you can put a machined flat part on that plate and check for twist and move it around your surface plate, to see if anything changes.
If you buy a vise and it’s not perfect, AND if you can detect that, there are things you can do about it: You can shim it, find a place on your worn mill table that compensates, tram your head into the vise instead of the table etc etc.
If you are buying from a source that doesn’t offer believable certs (anyth8ng whatsoever from China, Eastern Europe, South America, India etc) you don’t really know what you are getting. My advice, is to be skeptical and have a way to inspect. You probably won’t be able to inspect to the level of calibration. I think that’s kinda what
@slowtwitch73 was hinting at. But that shouldn’t stop you from trying. Because I can’t measure .00001”, doesn’t mean I shouldn’t measure .0001”.
Hopefully this helps somebody know what to look for and make a decision that is right for their circumstances.