Well, the -400 is the only of the 3 with 2 overwing emergency exits, and the -500 seems to normally have 8 windows between starboard front door and first gap in front of wing, whereas -300 seems to usually have 13, I believe.
That's what I have come up with as of now.
-500s are N5**SW, WN
-300s N3**WN/SW, or N6
**WN/SW/AA ,AS -400s are N7
**AS , Â
Also, UA -300s I have seen are N2**UA and N3**UA, and their -500s I have seen are N9**UA
[Edited 2006-02-27 03:15:33]