This will be the problem for the average person. If you have a protractor laying around, have someone hold the bike at the critical angle and stand across the room from it, eyeballing along the flat of the bike and line up the protractor with the floor and lean of the bike. I really can't think of an easier way to do this short of having some kind of jig made from cardboard that could be used to capture the angle quickly and then actually measured with a protractor once both hands are free again and the bike is safely propped against a wall instead of precariously leaned over.

You could even print out a protractor from the web to do this. Here's a pretty good scan of one:

http://www.dynapod.com/protractor2.jpg

I think designating fully upright as 0 degrees (as opposed to 90) is the most logical, so the higher angle you can achieve, the better off you are with regards to pedal strike. A tallbike can probably get 80 degrees by this measurement! Next up: find out what angle of lean causes a bike looses traction on clean, level pavement. This would dictate the maximum useful pedal clearance angle on flat surfaces. If you pedal strike due to hitting a curb or something, your mileage will definitely vary.

I'll measure my bike when I get home from work and probably start a thread for this in one of the main sections.