If you have an amp limiting power supply, throw 1 amp over the brush and measure voltage drop across it for ohms. I generally repeat at 5 and 10 amps and average out.
After trying to hunt down reliable suppliers for various different brushes, I decided to try some 20% silver brushes from Team Trinity. Hopefully they will be able to strike a balance between reducing comm wear without accelerating brush wear too much. I ran-in the brushes with the throttle trim turned up for a couple minutes, then took the car outside for a couple top-speed runs. My multimeter says the 20% silver brushes rate at 0.2 ohms from the face of the brush to the end of the shunt, whereas the Reedy Plutonium brushes rate at 0.05 ohms, but that slightly increased resistance wasn't enough to slow down the car I installed them in for initial testing. After 3 top-speed runs, the serrations haven't worn-down completely, so hopefully the brushes will have a decent lifespan. We'll see how my Traxxas Slash responds tomorrow; that truck has a Holmes TorqueMaster Pro 550 11t motor in it, and it's a brush-devouring monster; slightly higher resistance might actually be a good thing in that respect.