Well, you could always try to send them a Nudge/Wink.
Or you could like M_e said check the Client Id, fx ver. 6.0 uses MSNC1 witch is 268435456
6.1 uses MSNC2 witch is 536870912
6.2 uses MSNC3 witch is 805306368
7.0 uses MSNC5 witch is 1073741824
Clients like Trilian and Gaim uses Client Caps to identify themselvs