Use a stopwatch. Place a piece of masking tape on the platter at 6 oclock, and another piece at 6 oclock on the plinth, and yet another piece of tape at 1 oclock on the plinth. Put a record on, start the turntable, put the stylus in the groove. Start the stopwatch when the two pieces of tape line up at 6 oclock. At exactly 1 min and 40 sec the tape on the platter should line up with the tape at 1 oclock on the plinth. If it does not, you know what to do.