How an I confirm my Bluetooth 3.0 Keyboard is encrypted? I have a MacBook pro Retina 15, Late 2013 running Mojave. I also have an off brand Bluetooth 3.0 Keyboard ( GeneralKeys PX-4068-675 ). 

The keyboard connects fine to the MacBook WITHOUT asking for a pin ... but I'm not clear if the connection is using/requiring encryption.

Is here a way to verify it is encrypted?
I suspect you should assume your keyboard is not secure. Much depends on who is likely to be attacking you – prankster or nation state:


  While Bluetooth has its benefits, it is susceptible to denial of service attacks, eavesdropping, man-in-the-middle attacks, message modification, and resource misappropriation.


These two questions on related StackExchanges give a good overview:


Is Bluetooth 4.0 traffic encrypted by default/design?
How secure is a bluetooth keyboard against password sniffing?


Hopefully you can secure the environment in which the keyboard can broadcast.

WireShark

The WireShark tool maybe able to intercept the Bluetooth traffic. See Bluetooth Smart Wireshark Plugin for a guide. Also see Bluetooth Smart Security for more technical details.