For me, I usually "feel" the difference before I can objectively listen out and spot what it is. To use another analogy (it's just the way my brain works), It is like watching the same move on DVD and Bluray. The same stuff goes on, most of the time your attention is focused on whatever is going on centre-screen so the finer details pass you by, but when the occasional bit of shrapnel flies across the background in an action movie, you find your brain reading the serial number on it without noticing on the Bluray version. I "hear" a little scuff or line of acoustic guitar clearer on one IEM than on another, then go back and listen to the passage again to try and work out why I'm processing the sounds differently.
@warrior1975
wrote some interesting stuff on how he listens "critically" on the Campfire Vega thread a few weeks back if you can find it - he uses a Pink Floyd track with some PA style announcement in the background noise to test resolution on IEMs. It's little nuances like that in tracks you know well that make the difference -
at the end of the day, if the overall experience is fun, the other stuff is just the final 5% anyway!