Had this on Tumblr, so I figured I'd post it here. :P
...
The very first thing Dean Winchester does when his little brother walks through the front door of the rundown hideout (before he even has the chance to examine his appearance, to check if he's healthy and to into hug him into oblivion) is tackle him roughly to the ground and hold him there, pushing the overgrown man's arms back to the ground and pining the way too-long legs into the creaky floorboards with his knees.
Sam struggles and grunts on instinct, (Dean can't help but note that his reflexes aren't as quick as they'd been before he left- Sam would never have let someone sneak up on him so easily before) but then recognition sets in and surprise melds into shock. In the space of two seconds, Dean lifts the bottle of holy water and squirts it all over the man, and Sam- or the shifter or the demon or the leviathan impersonating Sam- gazes at him, bewildered and wiping away the water with his hands.
"I'm not a demon!" he says, and that much is obvious, because the wet skin does not sizzle and smoke. However, this fact far from deters the older Winchester, only rules out one possibility.
He picks up the next test- borax in a hard plastic bottle- and splashes it over possibly not-Sam's face with a determined vengeance, his chest tight with impatience. He's lived through Purgatory, through shifters and other evil things that could take a human form, and he'll be damned to escape just to find one of the bastards impersonating his only family.
"I'm not a leviathan!"
The borax, too has no reaction, no flesh melting off to indicate a black, gooey thing in disguise, and so he pulls out his silver knife for the final test and grabs a hold of his brother's arm. The cut is quick across the forearm, and with the final lack of supernatural reaction, he let's go and stands up straight, the relief flooding through his body not showing even the slightest on his face. It was Sam, actually Sam, not some bastard creature wearing Sam's face. If he hadn't just gone through a year of hell (and that was saying something, as he'd actually spent a whole lot longer than a year in actual Hell) he might have cried like a girl in a chick-flick reunion moment.
"Or a shifter. Good," he says gruffly, his voice not betraying him, and without hesitation, he lifts the two bottles up and points them towards himself. "My turn, let's go."