I've been reading up about higher framerate monitors and I've found that a lot of the 60Hz monitors can be slightly overclocked to show 65, 67, 70 or sometimes even 75 frames per second if you play around with custom display modes (people also seem to think this might damage your monitor, so beware and try at your own risk).
By increasing the framerate in Nvidia control panel I managed to get my 60Hz monitor to still show the image over HDMI connection up to 67Hz. And I have tested it in a couple games with unrestricted framerate and vsync disabled, but it doesn't show the actual visible framerate, and if I disable unrestricted framerate or turn vsync back on, it shows 60fps. So I couldn't figure out if it's really working, or it's a placebo effect and I'm just thinking it's slightly smoother.
I tried staring at the https://www.testufo.com/ animation, and the page says 67, but I don't know if it's just the browser thinking it's 67 and I can't count the frames without analyzing a video recorded with a high-speed camera. Some games are restricted to 60Hz or only show the closest supported graphics mode refresh rate, and again I couldn't determine with my bare eyes if it's true or not.
I tried using software like Fraps and Steam's in-game overlay FPS counter, and they either show uncapped game framerate (can be anything above 200fps) or 67, as the target framerate it gets from the OS display configuration. I also remembered that the monitor itself has its own OSD which shows display signal information. In my case, even though it's set to 67Hz in Nvidia control panel, the OSD shows 60Hz (68kHz horizontal) in some games (Dark Souls 3) and 67 (75kHz horizontal) outside any game.
These 2 tests (https://www.testufo.com/eyetracking and https://www.testufo.com/persistence) seem obvious enough to show what they're designed to show. Maybe there's a similar test to determine the exact displayed framerate?
Is there a way to make every updated frame per second obvious on the screen so that I could see it with my bare eyes? Maybe some optical illusion could be used or some special visual sequence of shapes, or a matrix of rectangles. I'm planning to buy a 120/144/165hz monitor sometime next year and I could use an answer to this question to determine if the monitor is truly displaying its advertised framerate.