The TRX will detect and recognize standard 15-bit NXDN encryption when the channel is stored in a memory channel, and it will also display the E symbol. Unfortunately it won't show an E symbol when the same channel is detected via a Limit Search (....and apparently Whistler support say that's not a bug !)
There are 3 settings for when it correctly recognizes Encrypted Digital Audio - digital noise, silence or tone. So you'd also need to have 'tone' selected to meet your criteria.