Generally, mildot scopes are correctly spaced at either 10x or the highest magnification if that is less than 10x.
It’s super easy to check though. One mildot is 1/1000 of the distance. So at 100m a mildot is 10cm. At 100 yards it’s 3.6”. Just measure out a known distance and make marks that are spaced 1/1000 of that distance on a piece of paper. Sight in on those marks and adjust your magnification until your mildots match your marks perfectly.
I measure all my subtensions at home (of scopes whose subtensions aren't advertised). I set up a yardstick from my gun room to the side of my bedroom mirror with a light on it and just do the calculations, and it's actually quite accurate. Could be done at the range too--just measure the target and range and do the math as Sawney wrote above. I also mark some of my SFP mil optics for the MOA magnification too.