My mom and I were watching a procedural murder mystery on TV, just the two of us, back in the early 1980s.

“Who do you think did it?” I asked.
”Olivia de Havilland,” my mom said, very confidently.
”Oh?”
”Well, why else do you get Olivia de Havilland?”

Sticking to that principle — who is the most prominent actor who isn’t the lead? — solves the mystery most of the time. Not always. But usually.