Try an ABX test: https://wiki.hydrogenaud.io/index.php?title=ABX
There's a reason MP3 is obsolete, though. You probably can't pass an ABX test with high bitrate MP3, but you almost certainly can't with AAC or Opus without extremely critical listening.
Agreed. My sound system isn't by any stretch audiophile; and I have the hearing of a 65-year-old. I can't point to things I can hear on a FLAC that I can't hear on an MP3; I hear the same notes, and the same instruments, with much the same tonality. But the former has a presence and vibrancy that is lacking in the latter; when I listen to MP3s, my involvement in the music rapidly tails off.
I suspect that you don't have to be able to hear higher harmonics in isolation, for those higher harmonics to affect what you do hear. That is, even if my hearing range cuts off at 12KHz, I can still tell the difference between an unfiltered sound, and a sound that is low-pass-filtered at 15KHz. The difference seems to be clearest with voices.