Sometimes it's more about knowing you have the highest quality format than being able to hear the difference. An mp3 of a great sounding album with good dynamic range will always sound better than a FLAC of a shitty recording.

I think most people can train themselves to hear mp3 compression even on low quality gear by listening to comparisons of cymbal sounds. An experiment to prove this is to import a lossless track in to a DAW, export it to mp3, import the mp3 and invert the waveform, so playing back you will only hear the differences between the two tracks, ie only the sounds that the compression failed to accurately replicate, the compression artifacts. What you will be hearing with an mp3-320 is a sort of muddy static sound whenever the cymbals hit, blended with whatever other vocals or instruments overlapped with that frequency. This doesn't mean that when you only hear the mp3 it will automatically sound bad or noticeably worse, but it proves there is an audible difference in the character of certain sounds that can be heard even on bad gear.