Hi Amanda:

Then photographing a line pair chart which can be purchased at a printer supply stores and better photography supply stores would be one way. Then find the least separtion that has not blurred together. The chart should be calibrated for distance but if not, knowing the distance and separation would allow you to calculate resolution in meaningful terms. One note; your optic is figured for infinity and the closer the target is to the mirror, the more over corrected it will appear and diminish the resolution.

Best,
Rick