Download a StrobeTach
the line voltage. Low wattage incandesent lamps also work. When the appropriate circle of bars appear stationary, the disk is turning at the indicated speed.
A recent Email suggests that on 50 Hz, the Strobe Tach works the same way, except that the speeds indicated change to 750, 1000 and 1500 RPM.
This StrobeTach will indicate 900, 1200 or 1800 RPM (and on 50 Hz), 750, 1000 or 1500 RPM