To add to Traxor, if a user has Messenger Plus! installed a "pong" message will come back. If nothing comes back, they do not have Messenger Plus!
This is the only way because Messenger Plus! respects a persons privacy
If you then right click a contact and view the deatils Messenger Plus! has stored about them it then will tell you if that person has Messenger Plus!. But you must /ping that contact first