[GraemeJ]

The only way I have found to do this is to enter the key manually.

By implication, it would seem that you are looking for a means of auomatically detecting the key - this is not possible!

You already have suggestions about how you might display the key, but whatever method you use (personally, I include it in the title, e.g. "Fly Me To The Moon - Eb") you are going to have to do it manually.