... I'm using this cool app on my iPhone... RPM by Philip Broder.
It uses the smartphone gyroscope and - well: it's for free and working flawlessly, precisely, safely... a true no brainer.
Just near to the spinning platter... no need of test discs, pilot tone frequencies, etc...
Just turn your cellphone ON and press the app button... and voilà!
That's it.
Love it.