In Men in Black: International, after Chris Hemsworth retrieves the weapon, he talks with Agent C at the after-party.
Agent C: I don't know how you do it, but you just keep on doing it.
Agent H: Yeah, whats that C?
Agent C: Saving the world from total destruction twice, in as many years. I mean, what are the odds?
Agent H: What are the odds?
They kind of look at each other and start thinking.
Then we see Agent C in High T's office, looking at the portraits of Agent H and High T fighting the Hive.
So how did C realize T was the traitor?