Sorry for the late addition.

It is indeed possible to check magnification accurately, however, it is dependant on your ability to force your eyes to do unnatural things. :)

You will need to be able to superimpose what you see through the scope over what you see through your other eye.

Magnification is indeed linear as you've described it. A yardstick at 10 yards is 10 times smaller at 100 yards.

So if you look at a yardstick at 100 yards, with a 10x magnification, it should be exactly the same size as one at 10 yards to your eye.

If you'd like to check this relationship out easily, get yourself a 10x and 8x set of binoculars and look at ruler about 10 yards away with one eye looking through each at the same time. Once you dick around for ages getting both focussed and pointing at the same thing, you'll be able to line up the bottom of the ruler. Then take a look at where the measurements line up. If you truly have a 10x and an 8x, they'll be overlaid at those measurements.

I've spent a lot of time doing this sort of thing with expensive binoculars to see if I can see any difference between different brands easily. One day I picked up an 8 instead of a 10 and then realised how handy this can be. This shows the magnification difference between two binos, but you CAN absolutely test magnification levels using one optic and two rulers at known distance relationships.

I recommend working out the math beforehand to make life easy though and using two identical rulers that aren't transparent plastic. You will also need to have the ruler far enough away so that you can see it without obstructing it with the spotter lens. It takes a LOT of dicking around, but once you have the position of the rulers, distance factor between the two matching the magnification to be verified and your head and eyes in the right position, once the image from either eye overlays exactly, you know that the magnification is DEAD on.