Cut something and measure how much force it takes to determine the sharpness. Cut a large amount of a specific type of material to blunt the blade. Repeat the first cutting to determine the sharpness. The force ratio can be used to indicate the amount of blunting. This is basically what the CATRA group does to a very high precision.
Note push cutting ability and slicing ability don't always correlate well they will both decrease as a blade is blunted but not always at the same rate. Soft blade materials with an aggressive structure can continue to slice well even when they push cut very poorly (as they have rolled) so the cutting to guage the sharpness should be done with both a push cut and a slice to get the complete picture.
The sharpness testing doesn't have to use a force measurement to put a number of it. You could for example take a sheet of photocopy paper (or whatever) and see how far out from the point that it is held that you can cleanly cut it. The ratio of the distances before/after can be used to represent the extend of blunting.
Or you could simply take a tomato (or whatever) and see how high up you have to drop it before the blade cuts it cleanly in two pieces. Use the height ratios before/after to guage the extent of blunting.
Concerning the material you are going to use to cut to blunt the blades, try to draw from the stock pile in a random manner. Ideally you want something that takes a decent amount of cuts to produce a blunting effect as otherwise material variances could skew the results too badly.
One final aspect is that how quickly the blade is restored is very important. So after you have tested the blade and determined the extent it has blunted spend some time sharpening it and periodically test the sharpness to see just how fast it responds to honing.
Some more detail here :
http://www.physics.mun.ca/~sstamp/knives/blade_testing.html
-Cliff
