So here's another method that doesn't rely on the song being detected or even a song at all: The gyroscope on your phone is accurate enough to measure RPM to a

So here's another method that doesn't rely on the song being detected or even a song at all: The gyroscope on your phone is accurate enough to measure RPM to about the first decimal place at 33 1/3 - 45 rpm's just by placing your phone on the turntable flat and letting it spin.