I'd follow Q.G.'s suggestion with a retrofocus lens and just measure the entrance pupil--the size of the aperture as it appears at the front of the lens. Put a ruler or calipers across the front filter threads of the lens and measure it, and you'll be close enough, and then use that value to calculate f:stops using the formula, focal length/entrance pupil diameter=f:stop (or more likely, you'll want to know the normal stops and use the formula to determine what the entrance pupil diameter will be for each stop).
If you hunt around on photo.net, Noah Schwartz used to have some illustrations demonstrating how to do this. It may still be there.