Dean: Cas, you got your ears on?
Dean: Listen, you know I am not one for praying, ‘cause in my book it’s… it’s the same as begging. But this is about Sam, so I need you to hear me.
Dean: We are going into this deal blind… and I don’t know what’s ahead or what it’s gonna bring for Sam. Now, he’s covering pretty good, but I know that he is hurting, and this one was supposed to be on me. So, for all that we’ve been through, I’m asking you… you keep a lookout for my little brother, okay?
Dean: …Where the hell are you, man?
It’s the second time in two episodes that Sam understood romantic feelings felt by characters without it being explicitely stated to him: firstly between Portia and James, and now with Artemis and Prometheus. Dean remained totally oblivious.
It makes me think… that if destiel becomes canon, Sam definitely already knows about it. He can’t not know. And he will definitely understand it before Dean himself, aha. :’D