When using s/mime when sending encrypted emails, there must be communication between the sending email system (the from address) and the receiving email system (the to address) prior to sending the information message between the systems.   This must be done to exchange keys.  The user (who is the sender) must have a visual indication that the message will be encrypted before hitting the send button, as this may influence his/her decision whether or not to send the email.

The S/MIME description only describes how to tell whether the message will be signed (wnich is not the same as encrypting the message to prevent bad actors from stealing information from the message en-route).  It does say that an icon will be present if the receiving system has implemented S/MIME.  Completely missing from the description is how the user can tell if the system is ready to encrypt the message before sending to ensure that the information isn't stolen en-route.  

Please explain how I as a user can tell whether or not the message will actually be encrypted prior to sending it.  This implies that an exchange of keys has been made between the systems and the the system will use the keys as part of its encryption and decryption processes.   

Thank you,

Robert