How do we verify magnification?

I picked up a pair of 30x60's and compaired to my 15x70's they seem more like 20x. Had some fun looking at the moon with one eye in each... They are made in China, J.C. Optics brand of the week, so I can't exactly vouch for the manufacturer's claim. They do work well though. So.... How do we check magnification? Any ideas.