Shoutbox

AFAIK, The only way you can know if a contact has Plus! installed is by using the /Ping command in a conversation with them.

If they have Messenger Plus! installed, then you will get a response along the lines of:  Pong! [2 secs]

However, I think there was an option somewhere in Plus! 3x that enabled you to disable replying to Ping commands (this has apparently been removed from MP!L), so it may not always be accurate.

Hope this helps...

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Some people may not want other people knowing if they have Plus! installed, hence why no 'invisible' method has been implemented (as it could be considered a breach of privacy), although it has been suggested before.