If asking your contact doesn't solve the issue, you can do as Scone said, use the Nudge (I do that
).
If they have Messenger Plus! installed, tell them to type (!VER), as it'll show their OS, MSN Messenger and Messenger Plus! versions (they must have Commands and Tags enabled).
As far as I know, the version number is not stored anywhere when the contact is writing to you or you see them in your list, so there is no way it can be known by Messenger Plus! or a plugin.
Edit: well, yeah, the protocol. But still, why mess with that, when you can just ask?