There are a couple ways to verify, but nothing 100%.
It's pretty obvious that the magnification markings are off a little on some scopes. For an SFP scope you can use the stadia spacing of the reticle to verify the magnification markings. I have to assume that the reticle is made to the correct size. For instance, scaled so that a mil-dot spans one milliradian (usually at 10x). If it only spans say 0.9 milliradian, then the actual magnification for that scope would be 10/0.9=11.1x.
For a standard (10x) mil-dot. Setup a ruler or yardstick at 100ft from the reticle (measure distance to the magnification ring which is near the gimbal/magnification ring).
If 10 mil dots spans 10” across the ruler then it is at 10x magnification.
If they span 9” then it is 11.1x
If they span 11”, then it is 9.1x
etc.
If you don't have a stadia style reticle, or you have a FFP scope, then you can use the advertised FOV at min and max magnifications and compare those values to what you actually get at different magnification settings.
Even when the magnification markings are not too accurate, most reputable manufacturers can get the reticle size and FOV values correct, since they are less dependent on tolerances.
Though it's also possible that a cheap scope has no accurate references. In that case, take a digital picture in/out of the scope and count pixels. Though that is more involved.
