When James stuck eye of newt in Lily’s hair in second year, and charmed her books to sing whenever she opened them, Sirius knew he fancied her.
When Evans responded to this by hexing James Potter, Sirius knew she needed some convincing.
When she burst through the portrait hole in sixth year laughing, with snow melting in her hair and James Potter at her heels, Sirius knew. He isn’t sure what he knew, but he’s sure he knew.
When Lily blushes a bright red after James talks himself in nervous circles about how pretty her eyes are, Sirius knew she was coming around.
When Lily gives James a hard time at dinner some nights, Sirius knows it’s because he’s sitting next to Susan Spinnet and not, as she tells him, “Because he’s being an arrogant prick today.”
When Sirius found the two of them asleep on the couch, the notes they had stayed up reviewing still open, James’ glasses crooked, Lily with her head on his shoulder, Sirius doesn’t wake them up. He knows. Even if Lily and James don’t know it yet.