Sam knew he was a good poker player; better than most if he was being honest. But even he knew he couldn't compare to Dean who really was at the peak of his game.
Dean's 'poker face' was the envy of anyone who had ever played against him. It was blank; completely impenetrable. Impenetrable, that is, to anyone but Sam.
Sam knew all of Dean's little tells. The imperceptible quirk of his left eyebrow when he had a couple of aces, the faint twitch of the nerve in his temple when he had a crap hand.
If Dean licked his top lip, he was thinking about folding. If he licked his bottom lip, he was thinking about calling, and if he wrinkled his nose, he was ready to raise.
Sam always knew if Dean had managed to snatch a crafty peek at someone else's cards, because he'd take a sip of whisky to hide his smirk.
And of course, there was that one time that Dean somehow managed to acquire a royal flush. His face had been impassive, frozen, unreadable; hands gripping the cards with a white-knuckled ferocity.
It was such a shame when he'd ruined the whole effect by drooling over the table.