What kind of TT are we talking about? If it is direct drive (no belt to slip) with speed set by a crystal oscillator the TT is probably more accurate than any device you can buy to measure it.
Measure My Turntable Speed
I've been told that a very accurate way to measure the speed of a turntable is by using a test meter set to khz, placing the positive terminal into one of the outputs of the phono stage (or tape output), the other to the ground and then play a 1000hz test track of our a good quality test record... If the table speed is good, the reading should be very close to 1khz...
Ever tried this one?
Ever tried this one?