Easy, here's how.
1. Find a location, preferably a quiet room, that contains the HE90, the HD 600, quality source and amp, and some great music.
2. Warm up the HE 90 and HD 600.
3. Put on some tunes and listen with the HE 90 and HD 600 in turn.
4. Notice how they compare.
The HE 90 is awesome-sounding, and vastly more detailed than the HD 600 sonically, but they are both natural-sounding headphones that seem to play anything really well.
After listening to the HE 90 and HD 600 it became apparent just how great the HD 600 perform for their price.