Honestly, not telling the difference is like being color blind. There's clearly something wrong with your eyes if you can't. Describing how 60fps is different from 30fps is like explaining how red is different from blue.
We can all say 60fps is "smoother" or red is "warmer," but those words could never fully convey what the difference truly is to some who's just plain deficient.
Honestly, I think that 99% of people who say they can't tell the difference, actually can and just don't know what to actually look for. I remember when I thought I "couldn't tell the difference." Then I played a shooter that was 30fps for the first time and hated how it felt. I did some research and found out that it was 30fps, and went back to play Metroid Prime, and had my mind blown that 99% of the reason the game had that, then indescribable, "feel" was just me playing a 60fps game. I played a bunch of other 60fps games I had, and it was clear. Nothing special. Just a game that runs well.
Once you see it, you can't un-see it. It's literally like discovering a new color you always were able to see, but had never thought about putting a name to. It's like "holy shit! The sky is blue! How could I have not noticed?!"







