Here's another subtle one, said shortly before Columbo even decides on a prime suspect....

"Blueprint For Murder"
Columbo, Goldie, and Jennifer Williamson are gathered in a room together trying to sort out why Bo (Beau?) has disappeared. When Goldie makes reference to Dr. Moss being her ex-husband's heart specialist, Columbo says "He had a bad ticker, did he?" "Did" is past tense, implying that Columbo suspects Williamson has been murdered. His suspicion was recent and justified because in the previous scene he took notice of that radio/tape player music discrepancy in the car. Moments later, Jennifer says that Elliott Markham saw her husband before he "left town," and the scene ends with Columbo pinpointing a suspect.