Up till now I have used strobe discs when checking turntable speed, I know some have other methods involving frequency counters..
Watching the latest Shango066 video on youtube, I noticed he has a far better way of doing this.
He stuck his android phone on the platter, started the deck, and low and behold, an app using the gyroscope in the phone gave a precise RPM readout
Not sure what the android version is called, but there is an iphone one called "RPM".
Apologies if this is a known thing, it impressed me and my strobe disc shall now be banished.
One word of warning though, make sure you have a decent rubber mat on the turntable. Sitting it on the metal platter your phone will flung away, at least at 78rpm anyway
.