I am open to suggestions regarding how to best view the rotor to look for proper tracking.

I've used black tape on the white rotors as suggested, different width pieces of tape, and changed one to red but I simply cannnot differentiate between the two rotors. I see some vibration at low speeds that moderate with RPM increase but don't feel comfortble that it is tracking.

I have considered shooting with my digital camera but doubt I can get enpugh shutter speed. Any other suggestions? Thanks.