They know each other so well.
Song Ji Hyo know where Kim Jong Kook usually hide. In episode 93, when other members hunted KJK, Jihyo knew exactly where did JK hide. JH said that JK would hide in basement or emergency stairs.
In episode Princess Ji Hyo, when Big Bang came as guest (ep. 163). JK chose JH's favorite dog, Bulldog, correctly.
JK always knows that JH will always rips his nametag in the end.