On a perfect display you should see just a faint grey circle.

Another test is moving cursor fast across the white page and tracking it with eyes. On a perfect display it should be perfectly crisp, on my display it blurs and moves in steps.

So basically on a perfect display you can track fast moving things, and when not tracking, they are blurred. On a bad display, things blur when tracking them, and you see several instances otherwise. For example, if you scroll a page with a black box up-down, on a bad display you would see several faint boxes overlayed, and on a perfect display one box with blurred edges.

The jump forward doesn't even necessarily feel that huge but the step backward is (annoyingly) noticeable.

However, I'm typing this on my Dell monitor which only does 60 Hz. It honestly doesn't bother me at all. Sure, when I scroll long pages I see the difference: the text isn't legible. But, in practice, I never read moving text.

However, one thing on which I can't go back is resolution. A 32" 4k screen is the minimum for me. I was thinking about getting a wider screen, but they usually have less vertical resolution than my current one. A 14" MBP is much more comfortable when looking at text all day then my 14" HP with FHD screen. And it's not just because the colors and contrast are better, it's because the text is sharper.