Blind test? HydrogenAudio forums will tell you that if it's not a blind A/B test you can fool yourself very easily.

You might also like to try AAC (or ogg) at a similar bitrate: MP3 has a couple of encoding peculiarities that are an unavoidable function of the format & if you know what to listen out for they're fairly obvious even at higher bitrates. (I'd still be surprised if you could tell the difference between flac and 320kps mp3 in a blind test though.)