Almost no one can, or have equipment that can make the difference. That being said, the difference between an old mp3 and a new one is sometimes noticeable at the same bitrate. The encoding algorithm has improved a quite lot since the late 90s. I bet a lot of people who say they hear the difference think about those old encodes.

I keep flacs as a master format that I make new encodes from when I want it on my phone and such though.