Another time Columbo uses that phrase is when he is talking to Adrian Carsini in Any Old Port, and he is wondering why there were no water marks on Ric's car. And Carsini asks him to let him know when he finds out the reason....and Columbo comes back with..."Believe me Sir, you'll be the first to know."
Another form of "You'll be the first to know" without using those exact words was in "Agenda For Murder" when Columbo tells Oscar Finch (Patrick McGoohan) "With your assistance, we can put this case to bed."
In "Identity Crisis", Columbo is looking at the
photos on the wall of Nelson Brenner's (Patrick
McGoohan) den. Columbo sees the photo of the
younger but balder Brenner stading in front of the
T-33 jet aircraft. Columbo doesn't say anything,
but he suddenly turns and gives a strange look
at Brenner, and Brenner understands what Columbo
has come to realize, that Brenner is really bald,
just like the mysterious Mr Steinmetz.
Not a single word is said, but Columbo conveys
to Brenner that he is on to him.
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.