1. James always knew about the feelings Sirius and Remus had towards each other.
He could always tell in the way they look at one another. It was like the way he looked at Lily, although their shy glances at one another probably came off a little less creepy compared to the way James sometimes stared at Lily.
James also knew the exact day when Remus and Sirius first got together. It was a Wednesday. He noticed how at lunch that day they were more hesitant while sitting next to each other, almost like they were afraid of being too close.
A week later, James and Sirius were in the common room working on their Potions homework when James decided to ask Sirius about it. After finally working up the courage, James casually leaned over his homework and whispered, "So, you and Moony, eh?" Sirius just stared back at him for a second before his face broke out into a big grin. "Yeah, me and Moony."