Measuring Turntable Platter Speed

I have an older Rega P2 that seems to warble (for lack of a better word) when playing. It's not that it's fast or slow, but I can hear slight, quick, pitch changes when playing a record. I put a new belt on, but no change.

Is there a method or relatively inexpensive instrument that I could use to monitor small fluctuations in platter speed?

Thanks in advance - Jim S.