Inspired by Holly-lemon on Tumblr's headcanon:
Lily figures out that Remus and Sirius are a thing before James does because he's James and oblivious and she brings it up casually over dinner and James is just like "wut" and then suddenly jumps up and gets all excited and runs to the gryffindor common room and hugs them both because he's been shipping them for years.
"I knew it!"
James Potter had not been paying much attention to what his girlfriend had been saying, too busy admiring the way her fiery hair framed her face, but this piqued his interest. "Knew what?"
She looked at him, a grin on her lips. "Alice and Frank are getting married. I always knew they would be a good couple."
"Hn." He twirled his fork in his mashed potatoes, thinking about it. Frank, who was a year older then him and had graduated Hogwarts the year prior, had once confessed to liking Alice but he'd never told James anything about the two being a couple. "Since when have they been a couple?"
Lily shrugged her shoulders, shoveling a spoonful of corn into her mouth. She swallowed before she spoke again. "It's news to me." James scowled. "Oh come on; you can't tell me you haven't thought which of our classmates would make a good couple?"
"Well yeah…"
She smiled. "See? I didn't know they were together, I just knew they'd be a good couple. Just like I think Marlene and Harold would make a good couple. What about you?"
James looked down at his plate, and mumbled something quickly. Lily cleared her throat and he sighed. "Sirius and Remus," he muttered, his voice barely above a whisper.
"Oh my God!"
James could feel his cheeks heating up. Red crawled across his face as Lily tried so hard not to laugh that it came out as a snort. "I know it's crazy," he snapped, "I just think it might be a good idea. You so aren't mentioning this to them." For some reason, that only made Lily laugh harder as she was no longer able to contain it in.
"I don't believe this," she whispered hoarsely. "You are so blind, Potter."
James frowned. "What?"
"They are a couple," Lily said as if it were the most obvious thing in the world. Her expression conveyed that same thought. James must've given her a look that conveyed his own thoughts (that she was insane) because she rolled her eyes and sighed, launching into an explanation. "Remember when you were younger? You and Sirius were inseparable. Since half way through fifth year, Remus and Sirius have been inseparable. And they're constantly undressing each other with their eyes; it's so bloody obvious. They 'discreetly' touch and bump each all the time, their hugs last way too long, and have you noticed them holding hands on occasion? You're an idiot not to see it."
James blinked, blinked, and blinked some more. Now that he thought about, it was obvious.
"Oh."
Lily slowly clapped. "Yeah."
"Oh," James said again. Lily shook her head.
Abruptly, James shot to his feet. The amusement in Lily's face dropped away as James took tearing off through the Great Hall. She called after him but got no response.
Up in the empty Gryffindor common room, Sirius lay with his head on Remus' lap, listening to the distant hooting of owls and the closer crackling of the fire. Remus had one hand resting on Sirius' hip, the other holding a book.
"You two!" a voice suddenly shouted. Sirius and Remus were on their feet immediately, eyes wide and mouths opened to give some well-rehearsed excuse. James just smiled. "You two are so cute." The utter confusion on his friends' faces just made his smile widen. "You guys make a cute couple," he said with a wink just as Lily pelted into the common room and tackled James.
And that was how James learned about his two best friends and their illicit love affair. Shortly thereafter, the whole school knew because of James' big mouth.