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He's literally cheating.
Seriously though it is mostly intuition. The logic he follows might come from counting the man tiles in his own hand against those in the pools. It's common to read discarding tiles that have potential to connect with terminals as signalling a completed terminal meld, or that one doesn't need a terminal meld at all. I don't remember the hand from the episode but I would guess Tetsuya holds many 1-2 mans. Your guess is as good as mine on how he guessed the guy on the right
