When sending an PGP encrypted message, one has to open the "options" menu to see if it is encrypted or not. There should be some marker to see if encryption and signing is active. Also for received messages the interface could be improved. Kmail shows a blue marging if a message is encrypted, green if correctly signed and the key is known, yellow otherwise, and red if the is a signature error. Maybe you could use a similar principle ...
Since 3.16.0 there are buttons in the Compose window, which clearly indicate whether signing and encrypting are selected. Since then it was no longer necessary to open the Options menu. Messages which are received are clearly indicated and checked by the bar below the message view, and the mime icons to the right of the message also clearly indicate the status. That is, all the indicators are there already.
Created attachment 1956 [details] compose window with encrypted message
Sorry, my comment was lost: The attachment in the last comment is my compose window for an encrypted message (freshly compiled version 3.17.3git101 on OpenSuse Leap 15.0) - I never saw any icon indicating enryption ...
Your screen shot does not show an encrypted message, it shows a message which is going to be encrypted. When I said, "Since 3.16.0 there are buttons in the Compose window", that was not 100% correct. They are available since 3.16.0, but you must add them via /Configuration/Preferences/Toolbars/Compose Window.
Ah, great, it works! Thanks! No so easy to find :-(