You might be able to devise a test. Try these things out. Count in your head at a steady pace using words and try to hold a conversation at the same time with

You might be able to devise a test. Try these things out. Count in your head at a steady pace using words and try to hold a conversation at the same time without losing your timing. Then imagine a moving image of the same numbers steadily ticking over and again try and hold a conversation without losing your timing. The 2nd scenario should be easier. You can do similar things with memorising numbers. Next time you have to enter your OTP, sound out the digits to memorise it before entering it. Then try memorising a picture of the numbers before entering it.