It's usually pretty trivial to detect MP3 vs. uncompressed with the benefit of a wave editor, since the steep discarding of frequencies above 18K or so is usually a dead giveaway.
Telling the difference with ones ears, especially considering the quality of modern encoders, is significantly more difficult, even at 128kbps.
I'd bet money that using LAME 3.98 and 256kbps, you'd need to rely on one or two VERY specific passages of music, "killer samples", to get anything out of range of a coin flip on a DBT, regardless of the equipment you were using. The technology is really that good, and the things MP3 will discard to make the extra room are really that imperceptable to mortal ears and brains.
At 128kbps, you'd have a better success rate, but I think people would be surprised at how often they wouldn't be able to tell one from the other.
In my experience most if not all of these very rare killer samples, interestingly enough, have been from very heavy, industrial-type music, things with lots of synthetic sounds and "buzzy" waveforms that give the encoders a run for their money. Classical or other "organic" types of music, for lack of a better term, actually compress very well, and at higher bitrates are going to be audibly indiscernable from the source material to anyone without a Cool Edit Pro license.
