Is it ever explained by Harry how he knew about Whiskey's nature in Kingsmen: The Golden Circle? I feel like I missed a line or two or something.
I think it was never explained properly but Harry did say Whiskey made Eggsy drop the cure on purpose. I didn't suspect it as it looked as if he was really trying just to save Eggsy from the shooting but i guess Harry went by his intuition. I also don't recall any further explanation from his side.
Watch the movie again. Harry started to suspect and give Whiskey questioning looks the minute he pushed back against Champ stating the president was wrong to not help those people Poppy infected. Whiskey said, "Sir, we can't let this get personal" that pushback was enough for Harry to start to suspect he had other motives. He kept a close watch on him after that and he finally reached his conclusion when Whiskey knocked the vial out of Eggsy's hand.