Using your phone or tablet and some program for sound analysis (like "FFT" = Fast Fourier Transform) you can simply put the terminal close to the propeller rotating and "heard" the noise. The software performs the FFT of the noise and shown main vibration mode frequency, that is just the one of the two blades rotating.
Image below shows the FFT of a motor rotating at 5700rpm, giving 193 Hz = 193 Hz x 60sec/min / 2 blades = 5700rmp.
In other words, for a two blade propeller, simply multiply by 30 the Hz value.
Hope you like this tip
Angel