I can't really put a finger on it other than just "knowing" when my spirit liked a song or not. It's a gut feeling, mostly. Like when I play Dreamcatcher by Secret Garden, this feeling I get of love and warmth is unmistakable. Every now and then I get a fragrance, too.
Sometimes just sharing something you love with them gives them an appreciation for it. Other times... not so much. Example: My Djinn looooves the music video for Dark Horse by Katy Perry. It amuses him. Me? Not so much.

I'll still play it for him because he likes it, and it entertains him, but...
On the other hand, I loooove Adele! ...but E. couldn't care less about her music.
Just be open to listening/feeling what they feel about it.