Go to mp3ornot.com and take the test using the best headphones you can. See if you can tell the difference between a 128kbps and 320kbps mp3 file. They use a random selection of music, some of it's a little disingenuous as they pick some tracks that would tend to mask compression artifacts, so this is a real challenge even for an audiophile. I did it on my work computer using Dell on board sound and my HD201's, a $15 headphone. Nailed 3 out of 3 on my first try but it was difficult. Some tracks I listened to literally 20 times or more, concentrating on different instruments, their dynamics.
What you want to do is understand what mp3 compression artifacts sound like, and where they might present in a mix. It will be more audible on higher frequency percussive sounds like cymbals, and it affects the stereo signature, so panning percussive sounds are even better for critical listening. Try to find something in each song that fits that description, and concentrate on it like your life depended on it. Try to find any difference in that sound from one to the other, then compare to X. Then post your results!
Here are my results