9th Doctor:
He knew he loved you when you said you trusted him. It wasn't even a life threatening situation. He just asked you of you trusted him. Yes you answered instantly. He knew you would trust him blindly.

10th Doctor:
He knew he loved you when you were the only thing that kept him grounded. He was starting to go mad but you pulled him back. You heal him.

11th Doctor:
He knew he loved you when you were captured by the silence. He didn't knew what do . His mind went crazy, his hearts nearly exploded. Amy even pulled him aside to tell him to take a deep breath and calm down. When he finally calmed down he realised he loved you and had to get you back.

Rory:
He knew he loved you when you chose him and not the Doctor. He was dumbstruck. You left the Doctor, the time lord, two hearts for him the normal big nose human... wow

Mickey:
He knew he loved you when you gave him a chance. He knew what your old boyfriend had done and knew it would be hard to accept him but you did. You gave him a chance and he took it with all his heart.