The quantitaive answer in the evaluation of a working fan, would use the technique called current ramping.
Current ramping is the plotting of the instantaneous current over time with an oscilliscope. It would be done mostly with an inductive amp probe. Using such a technique current rises as the brush enters the commutator post. It builds till it leaves the post. This happens quickly but is easily caught on a scope. This technique is usually used on fuel pump motors and a quick google of "current ramping" shows this example:
http://www.wrenchead.ca/HTML%20Presentation%20folder/sld111.htm
The amount of current, the uniformity, and individual sector issues can be addressed.