Technically, there is no way to just "know" if they have Messenger Plus! without some server interaction, probably where Patchou would have to store a users passport address in a database and have Plus! access that database.
However, he could maybe add more information to the Ping, like, a version number or something. But I dont think it'd be a very good idea, it's too much extra programming when you can just tell your contact to get MsgPlus.
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