Sam keeps seeing the ring box in Dean’s duffel bag, on motel nightstands, even in Dean’s hands when he thinks Sam isn’t looking. He knows there’s an actual ring in there because he couldn’t resist peeking inside once while Dean was out. It looks beautiful and expensive and Sam just immediately understands.
It takes almost six months of catching glimpses of the same ring box until Sam has had enough. He picks it up from the stuff splayed out on Dean’s bed while they’re packing.
“Dude. Don’t you think this thing would look better on Cas’ finger than in this damn box?”
Dean blushes furiously and snatches the box back. He doesn’t speak to Sam all day.
But apparently it was all that was needed, because next weekend Cas is in the bunker, showing off his engagement ring to Sam who has to feign surprise because he has already known about the stupid ring for so damn long.