I have a friend with a Achromatic Diopter that he picked up at a flea market, but we have no idea how strong it is - are they ways of telling?

You should be able to tell the focal length by going out on a sunny day, hold the diopter above a surface and try to make a hotspot (the smallest you can make) from the sunlight. When you've got it measure the distance between the (middle of the) lens and your object. Since you can safely assume the sun to be infinitely far away, the distance from lens to hotspot is also your focal length.

Then you divide 1 by that distance you found (in meters of course), and there you have your diopter strength! -example: if the distance is 200mm=0,2m , diopter is 1/0,2= +5

Have fun,

Steven

PS: Didn't you burn little pieces of paper as a kid? (or spiders - :O oh no I didn't say that :P )