Strobe
One way to check your tach is to use the strobe effect which you will get from a flourescent light. You can check your rpm at sub-harmonics of 7200 rpm. This relies on the fact that the light output from the flourescent is not steady but pulses at 120 pps. An incandescent bulb will work, but just barely, since its light output is steadier than the flourescent. This test is best performed in a shady area or just after sunset. Night works best if you are in a lit area so that there is no safety problem when working near the prop. You might even be able to observe this from sodium-vapor or similar lamps providing apron lighting! At these sub-harmonics the prop will appear to stand still with varying numbers of blades. For instance, a two-blade prop will appear as four blades spaced 90 degrees apart and standing still at 1800 rpm. At 1200 rpm it will look like a six blade prop, with a blade every 60 degrees standing still. This is a fuzzy looking blade, not a sharp ouline, but it's the relative motion or lack of it that counts!