I use the following method to check for speed accuracy. I can't say whether it's more accurate than any other method, but it is free and only requires a stop watch and some time.
Place a line on your turntable platter and place another line on the turntable's top surface. The lines are used to count platter rotation. Start the platter in motion and using the stop watch observe the number of rotations the platter completes. In 3 minutes you should observe exactly 100 revolutions; in 6 minutes 200 and so forth. Six minutes is probably the minimum time needed for an accurate measurement.
This method can also be used with a record playing.
Place a line on your turntable platter and place another line on the turntable's top surface. The lines are used to count platter rotation. Start the platter in motion and using the stop watch observe the number of rotations the platter completes. In 3 minutes you should observe exactly 100 revolutions; in 6 minutes 200 and so forth. Six minutes is probably the minimum time needed for an accurate measurement.
This method can also be used with a record playing.