In Episode # 1,
You may notice Beak Si Yoon kept marking every customers who hang out at the bar.
With his experience he can tell who were a spy and who were regular.
If he saw a spy he will mark them in his smartphone with " O " as Yes ( They are spy )
And when he met Choi for the first time and he wanted to test Choi by pretending to slide him
a glass of water to check his reaction. But Choi had no clue and think Beak is just a stranger
who acted too friendly ( Or weird) to him for the first time!^^
And that when Baek marked Choi with " X "
Mean he's not a spy! Hahaha
Credit : parksihoobar.com, thank you!