I don’t know for certain, but I know

• Cas plays with Sam’s hair. A lot. But especially when Sam is sleeping.
• Cas tends to lead when they walk anywhere together. Cas tends to move much faster than Sam does, despite his height and stride length disadvantage. 
• Cas has taught Sam all the dirty Enochian phrases he knows. They translate terribly into English (that whole “you breed with the mouth of a goat” thing is essentially calling someone a goat fucker in English) so Sam has given up on translating them because it totally kills the mood if he knows what the hell Cas is saying in bed. 
• Sam is not above doing that obnoxious thing tall people do where they put their elbow on your head and use you as an armrest. 
• Sometimes Cas forgets that he’s holding Sam’s hand so he’ll go to pick something up with both hands and Sam needs to jerk his hand away or get his fingers squashed. 
• Sam’s got a priest kink. Cas notices. (He tends to volunteer to play the undercover priest role on cases.) 
• They don’t actually have a lot of sex. Cas could take it or leave it but he enjoys the intimacy. Sam does not enjoy bottoming all that much. Frottage is their go-to. 
• Cas likes to tell Sam about the things he’s seen in human history. Sam records it and is planning on transcribing it into a book. 
• The actually spar quite a bit. Cas is quick on his feet and he keeps Sam on his toes. This has a lot to do with the fact that they want to exercise together (yeah, they’re that kind of couple) but Cas thinks running is a waste of time and he won’t get out of bed for morning yoga. 
• Sam does not like it when Cas makes coffee. He’s all for strong coffee but the stuff Cas drinks is strong enough to wake the dead. 
• They both have nightmares. They’ve both become each other’s rock in the middle of the night. 
• Cas is a bit of a mother hen with Sam. That’s probably because Cas can actually talk Sam out of doing stupid and dangerous things, unlike some Winchesters they could both mention. 
• Sam is actually more crafty than Cas, but that’s not saying a whole lot. Sam look a couple of knitting classes because he wanted to figure out how to do it. (Can’t be that different from sewing up a knife wound, right?) Cas tried and then gave up half-way through a lopsided scarf. 
• Cas actually has a lot of hissy fits. He’s very particular about the way he wants and likes things and really dislikes modern inconveniences. (They got stuck in traffic once. Cas huffed and rolled his eyes and gripped the entire time.) Sometimes it’s cute, sometimes it’s annoying. 
• Sam doesn’t like texting, but Cas does. Cas will send him texts written entirely in emoijs. Sometimes Sam knows what he means. 
• They don’t talk about hell or Lucifer. You’d think they would be able to, considering they’ve both been vessels of the same arc angel, but that’s the big reason they don’t talk about it. But they know when the other is thinking about it or having nightmares about it. 

I don’t know it for certain, I just know it’s true.