Cooper's first suspicion that Amelia has intimate feelings for Wolf Edmunds in Interstellar comes when they are on Endurance and Amelia is introducing him to the 3 planets that could sustain life. The conversation goes
Amelia: Laura Miller's planet is first. Laura started our Biology program.
Amelia: Ah... Wolf Edmunds is here.
Cooper: Tell me about Edmunds.
It seems this is the point of first suspicion. Why did he interject when Amelia would have given a brief background of Edmunds as is.
Amelia: Oh... Wolf's a Particle Physicist
Cooper: (judges Amelia's body language) None of them had families?
Amelia: No, no attachments. My father insisted. They all knew the odds of seeing another human being again. I am hoping we can surprise at least 3 of them.
Cooper: How about Mann?
Amelia: Dr Mann? He's remarkable. He's the best of us. He inspired 11 people to follow him on the loneliest journey in human history. Scientist, explorers. (getting emotional) That's what I love. You know, out there we face great odds. Death, but... not evil
By the way she describes Mann, I would think, that too as a wildest imagination, that Amelia had feelings for Mann.
Cooper than goes on to ask TARS, privately about them two.
Why did Cooper suspect Amelia had intimate feelings for Wolf Edmunds from this little conversation. How the hell did he suspect would be a followup question.