Stack clues that honestly look bad
A suspicion story only works if the reader would worry too. One clue is a coincidence, so stack a few that point the same way: a name said in their sleep, a notification that says “don’t tell her”, a red mark on a collar.
Write the accused person’s first replies a beat too short, the way people sound when they are guarding a surprise. The reader should build the wrong case from real bricks.