Here is the information I was referring to above. I found it in the Wiki.
One pretty simple way but not as accurate and one more complex but should point you in the right direction.
A quick first pass at determining runout is to simply cut a slot and measure it and compare it to the endmill diameter.
Alternately, cut a 4" square out of the material in question, then measure both the square and the resulting hole — half the difference is the effective cutting width, less the endmill diameter is runout.