You can also shoulder surf the actual measurement at the eye doctor. Wait at the eye doctor until they do the preliminary check of your eyes (you stare into a machine that clicks mysteriously and flashes lights in your eyes.) That usually measures your PD (distance between your pupils.) Then, when you notice they get a little printout or something on your file, ask what the machine did, and then ask to see the printout for a second. It should have some rough numbers of what your prescription is probably close to. If you play 'wow-ee, lookit all those numbers', you can be furiously scanning the little paper, looking for that precious 'PD' number, and noting it down. It's usually between 55-70 or so, and is measured in mm.
I guess your mileage may vary on this, but it's criminal of them to ask for like $100 extra just to get your PD (what my eye doctor was asking for.)