Shoutbox

Actually, simp (a Messenger encryption addon I suggested earlier in this thread) has no trouble detecting whether the other user also has it and handling the key exchange, completely transparently.  There are no visible "pings" or any other indications, regardless whether you have it or not.

If you don't have it and I do, it has several ways to alert me that our conversation won't be encrypted (including tray notifications, changing your text color and appending an indicator to your nickname).  You don't see anything.

If we both have simp, the first time we message each other it will present us with the signature of the other person's key and ask to accept or decline.  It will detect and warn me if your key ever changes -- that way I can know if someone else has stolen your account, as they would be using a different key.