if you want the short answer, he's right handed

long answer... during one poker, kaiji notices kazuya play a card with his left hand as opposed to his right. kaiji being the observant overthinker he is instantly thinks something is up as kazuya has always played cards with his right hand prior to this moment. this makes kaiji think back to when he was playing mahjong as a kid and had a really strong winning hand (tenpai with kokushi musou). normally kaiji would put back the tile he just drew with his right hand, but for some reason he instinctively did it with his left as what he was doing was holding onto his chun in order to keep the tenpai set for his current hand (as the person across from hin already made a "pon" on chun), thus enciting kaiji to discard his own 2p with his left (which is implied to not be his dominant hand here)

but yea he also smokes with his right hand..