There are several episodes where Columbo suspects the killer right from the first meeting; then there are others where he sort of plays around with the idea in his own mind. For instance --

Dr. Mayfield - suspected right from the start of "something" because he was resetting his desk clock.

Brimmer - and the palm reading incident

Franklin and his open mail.

There are more I'm sure.

Here's a few that he is seriously giving consideration to open first meeting --

Kay Freestone -- the way he keeps following her around on the first day he must have thought something wasn't right.

Dr. Flemming - he did not call out to his wife upon entering the apartment.

By Dawn's Early Light -- I think he always knew it was Col. Rumford.

Paul Gerrard in Murder Under Glass.

The fact that he never checked into a hospital after he was told of his dinner companion being poisoned.

Columbo even tells him "I suspected you from the first moment sir" or something like that.

One he discovered that Elliot Markham liked classical music he was off and running.

In Try And Catch Me, he evidently suspects SOMEONE, because of that "I doubt very much that this was an accident" line. It always bothers me a little that he "shows his hand" so early by saying that.

He not only shows his hand but turns the tide immediately, and she never recovers. I appreciate that part a lot, but it does tend to affect one's enjoyment of the mystery part of the episode. The interaction between the two compensates for sure. Murder Under Glass may be the earliest, but how many viewers picked up on it, until what Columbo says later, the finest example of good citizenship, or something along those lines?

I believe in "Troubled Waters" he had Hayden Danziger in his sites quickly when he saw the feather from the pillow in the hospital corridor. As he explained during the denouement, they don't use feather pillows in hospitals due to allergies.

His first big blunder was in expecting his wife to pack those gloves instead of doing it himself, and not relaxing till he actually saw them in his suitcase. If you're going to murder your blackmailing ex-girlfriend, don't expect your wife to help!
If he'd done that, the pillow business wouldn't have come up.

Columbo had to be pestered into hassling johnny cash at the beginning of swan song

In Grand Deceptions, Columbo does two things that he also did in Murder Under Glass (the only other time that I can remember). First he tells the suspect that he dislikes him. From memory:

MUG: I admire your talent but I don’t like anything else about you.

GD: You know, Colonel, the way we always agree with one another, that's amazing, considering the fact that we really don't like one another.

He also tells them that he suspected them right from the start:

Paul Gerard is a suspect because he didn’t go to the hospital when his dinner companion died of food poisoning.

Col. Braillie is suspected because Columbo thought he was too arrogant to be cleaning mud off the floor.