From an old book, Fun with a Pencil by Andrew Loomis, I learned how to create accurate shadows for invented scenes without photo reference. I teach the technique in one of my illustration classes, sometimes to the chagrin of students who try to make it way more complicated than it is.
I used the system in this early illustration for a magazine article on private chess clubs in New York City. Above each table is its own light source. The sconces on the wall woudn't provide enough light to interfere with the lights above the chessboards.
I've used this method a lot, but not all the time. As I tell the students, it's good to know it even if one doesn't use it.