You won't be able to if you're listening through most computer speakers.
Get a good set of cans or burn a 320kbps mp3 sourced CD and compare it to the same album sourced from FLAC on a true hi-fi system and you will be able to tell the difference.
If you have an iPod play a song at high enough volume (don't over do it like me) and then play the 320 and listen to the difference. I can hear the difference and I'm one deaf son of a bish!
320 and lossless are hard, 192 and lossless is noticable. Everything regarding bit rates is in general though, you have to try it yourself and pick what you like. My ears find 192vbr lame to be good enough especially counting in the space savings.
