They weren't much ones for public displays of affection, but Sam knew all the tiny gestures.
Cas sometimes wore Dean's shirts. Not in the patchwork motley sort of way he'd worn both Dean and Sam's things in the first weeks after they'd found him after the Fall, but with a quiet pride. He held himself differently when he wore Dean's shirts, as though he was more aware of the fabric against his skin, of the very visible symbol he was displaying by wearing them. Sam wasn't sure whether Dean noticed the subtle change in Cas's demeanor, or if Dean just liked to look at Cas wearing his clothes, but Sam definitely noticed the way Dean's eyes lingered on the fallen angel on the occasions that Cas opted to dress himself from Dean's duffel rather than his own.
The pattern of the beer was something Sam was rather proud of himself for figuring out - the beer cycled, depending on who had done the grocery run that week: when Cas had done the shopping, it would be Dean's favorite amber in the fridge, but when Dean shopped, it was Cas's lager of choice.
When Dean drove, Cas got shotgun. If you didn't know Dean, you didn't know the kind of importance Dean placed in who got to sit next to him as the miles rolled away under the tires. But Sam knew Dean, and had been in that seat for years; that Cas had usurped him wasn't cause for jealousy at all. (There was, surprisingly, more room for Sam to stretch out when he had the backseat to himself.) Instead, it was cause for a silent celebration that his brother had finally begun to venture outside the narrow, self-imposed definition of himself that wound so tightly around Sam. He worried about that occasionally; worried that Dean wouldn't know what to do with himself if Sam was gone. But with Cas in the front seat, he worried a little less.
And when Dean's hand was not on the gear shift, it rested lightly upon Cas's knee - another small gesture that was impossible to miss. Sometimes Cas would reach out and lay his hand on top of Dean's. They didn't give each other lingering, sappy looks when they touched like that; it was almost as though Dean was confirming to himself that Cas was still there, and Cas was reassuring Dean that yes, he was. Would always be. If they were on a long stretch of highway they'd stay like that for hours, until Dean had to downshift and he moved - but his hand always returned.
Sam couldn't point to any one thing and say "look, they're in love." But he also couldn't point at any one raindrop and say "look, it's raining." It was true nevertheless.
