Sam doesn't notice, at first.
And then he does, because it was kinda hard to miss.
The first thing he notices is that the guitar is out more often than not.
Then he starts noticing the small things. Like, Cas knowing more about music than he ever did. Or, Dean and Cas disappearing for long periods of time, though that could mean something else entirely and Sam just really doesn't want to think about that kind of stuff. Ever.
He also notices the really domestic stuff. Like, how Cas knows exactly how Dean likes his coffee. Or, how comfortable they are in complete silence, doing their own things, as long as they know where the other is. And the occasional movie nights, where they either completely lose interest in the movie or they fight over opinions or they just cuddle and one of them fall asleep on the other.
The fights are also indication. They are awfully domestic, and they are full of exasperation and fond, and another emotion that Sam knows is there but he's not sure his brother is aware of it.
It's the really sweet stuff, that catches his eye. How Dean will linger his eyes on Cas, smile cockily and say "what about a goodbye kiss?" before he goes out on a hunt. He always did get that kiss.
Or the relieved way Cas embraces and clutches at Dean when Dean goes through a near-death experience. And vice versa. The fury in Dean's eyes when someone tries to hurt his angel.
The morning routine, where they just work perfectly in sync with each other so it looks like they've been doing it for years.
Sam still remembers the time he walked in on Dean playing the guitar and singing "Thank You For Loving Me" to Cas. The way Cas looked at him spoke volumes. He still remembers the way Dean closed his eyes and sang "I never knew I had a dream, until that dream was you" only to open them again and look straight into Cas' eyes when he sang "when I look into your eyes, the sky's a different blue".
Sam then left. It felt like he was witnessing something private. Actually that's not right. He knew he was witnessing something private.
He remembers watching their relationship grow. From stolen glances to outright staring, complete with heart eyes and all.
Maybe at first he wasn't sure what he was witnessing, but he sure as hell knows now.
Sam just knows.
It's the way Dean and Cas stand close by each other, even when the room is completely empty. It's the way they always seem to touch. It's the way Dean immediately knows if Cas is in the room. It's the way they lean in to each others touch, however brief.
They are Dean and Cas. Cas and Dean.
And they are in love.
You could see it in every look. Every touch. Every word.
It was just them, and them completely.
Anyone who says miracles don't exists is a complete fool. As they are the perfect proof to it.