I didn't mean the headphones, I mean what powers the headphones. The headphone amp. You can't tell if you haven't heard a good brand (like Creative), but the iPod's headphone amp and sound processing suck.
For example, you would expect an AIFF to sound better than an MP3, right? Well, an MP3 converted from an AIFF, will sound better on a Creative player, than the origional AIFF on an iPod.
Also, to tell the difference, you will need some good headphones, for example, Sony Nudes. Yes, the name is weird, but they are one of the best sets of headphones on the market.