Most accurate IOS app for turntable speed check

Your ears, just admit, if you cannot detect that the speed is off:

IT DOESN"T MATTER!

I occasionally accidently brush against the wheel that controls my TT81's speed, and if just a speck, I don't notice, a bit more, huh, somethings wrong, oh hey, look at that number in the tiny window.

My Thorens TD124, (and my current belt drive Mitsubishi Vertical TT LT-5V) reflected dots, I would wait a few minutes for it to get naturally warm, adjust for steady, and then, if the room temp changed, say a party with lots of people, I would hear it wrong, check, sure enough the speed was off.

To a point, it only matters if you can hear it. My old ears have lost a lot of high frequency hearing. That said, I'm very aware of pith or wow. I play acoustic guitar and tuning is a constant chore, and I can always tell when a string goes off tune,

So, my trusty old Technics SL-1200 MK2 has the built in strobe that seems to work well and I've never detected any offensive pitch or speed variation.

Again, I thought the subject is how to measure speed, not how to maintain correct speed.  I agree with Elliot; it is not so important to maintain bang on 33.333 rpm as it is to maintain constant speed. I think we/I am more sensitive to speed varying during play than I am to slight errors in constant speed.  But the OP wants to measure speed per se.

Very easy to do with any laser tachometer you 

can buy online. You need some reflexive tape. Should be more accurate than an iOS app.

OP was asking about recommendations for "Most accurate IOS app for turntable speed check". Well, as @dover correctly pointed out, these apps are not accurate:

RPM and other apps I have tried are not accurate. Results are inconsistent. They are only good for entertainment - not calibration.

Just for giggles, I once installed RPM on two phones, then placed both phones on a spinning turntable with the apps running. The speed readings were different. What’s worse, the delta between the two readings was not constant.

My uneducated guess is that the inaccurate readings are a function of the phone hardware rather than the app itself; but in any event, these apps are not fit for calibration.