Whenever I hear someone proclaim that they can hear the difference between MP3 and uncompressed audio, I ask them whether they did an ABX (so-called “double blind”) test. When they reply “a what?” I have my answer. 😉
If you’re curious about your own ear’s abilities to discern different audio qualities, have a look at this page from npr.org: “How Well Can You Hear Audio Quality?”
Yes, it’s tricky – because you don’t know which is which. And that’s the whole point of an ABX test.