Measuring turntable speed

+1 on the RPM Speed and Wow app I used it on my TT and it registered 33.35 and I compared it to my $15.00 laser RPM meter that big_greg mentioned and that registered 33.29 so between the 2 I should be pretty close and my Acoustic Solid 111 Metal keeps it constant, I check it once a month and for 11 months no variation from my base line readings.

KAB speed strobe.

An electronic frequency counter. Used doesn't cost much. I like old HP stuff like the 5314. Just plug into the output on your phono/pre.

Lab accuracy, repeatability. Easy to get real 6 or 7 digit resolution. These days you can get used pro gear for chump change.

All the strobe things is what they are. 

But it lacks the resolution 3150 Hz (number/second) of  and the industry standard of DIN IEC 386 (formerly DIN 45507).
Like the "Feickert Platter speed" app.

I it is not in android app store (or Apple store). I just downloaded the app from here and side loaded it on android. 
(It side steps security so there is a risk. But you all have a old android phone laying around that can be used for this highly important task! ;) It will cost you $0.)
https://apkpure.com/platterspeed-vinyl-tool/ch.progtec.PlatterSpeed

Not just it has better resolution and graphs it also do the measurements exactly as when you are playing a record.
With the mass from it and you can use the clamp you usually use. And with the stylus in the groove with its tiny drag. 👍
In comparison I have never played a record with a phone on the platter. I see those low resolution apps more like a party trick and nothing when I want accuracy and the speed fluktration. (But that is maybe just me)

Just a discarded android phone and a record with a 3150 Hz track and you are done.

@optimize, how does one know if the 3150 hz tone was cut by the lathe at exactly 33.3333 rpm?