Sometimes it's easy to differentiate. If there's a dance room and the dance coach is not with a second sprite, but she's still working with another person; it means she's working on a song or concert. Same with the vocal coach. If there's no idol with them, but there's someone in the studio, they're prepping for a release. You can hover over your portrait, for example, to see if he's preparing for a date with an idol, or writing lyrics for a song.