Matthias at WoodGears.ca ran an interesting experiment a while back.  He used a simple test file to measure the kerf width when cutting with a laser.  The idea is to cut fingers out of a part, and then by using a dial indicator we can measure how much slop there is between the parts.  In addition he flips one half of the part over to get a rough measure of how much taper there is to the kerf.  Or more exactly to measure how close to the center of the piece the focal point is.

He later had another user re-run the test with a better calibrated laser, this really shows the genius of his test.  The new machine is not only cutting sharper, with less  angle on the cut but the cut is much cleaner as well with minimal scorching.

Anyway this is a really neat idea, I will try to replicate it on my machine.