知人知面不知心

zhīrénzhīmiànbùzhīxīn
one may know a person for a long time without understanding his true nature (idiom)

CEDICT

one may know a person from the outside but may not understand their true nature

Wiktionary

（谚语）仅看到人的表面，不能洞察人的真心。指对人难以真正了解。

重編國語辭典

仅看到人的表面，无法洞察其内心

維基詞典

Entry Composition

1 知人 zhīrén
  1. (archaic) to be an insightful and astute judge of character
  2. to be an insightful and astute judge of character
2 知 zhī
  1. to know
  2. to be aware
=3 面 miàn
  1. face
  2. side
  3. surface
  4. aspect
  5. top
  6. classifier for objects with flat surfaces such as drums, mirrors, flags etc
=3 面 miàn variant of 面 miàn
=3 面 miàn
  1. flour
  2. noodles
  3. (of food) soft (not crunchy)
  4. (slang) (of a person) ineffectual
  5. spineless
4 不知 bùzhī
  1. not to know
  2. unaware
  3. unknowingly
  4. fig. not to admit (defeat, hardships, tiredness etc)
5 心 xīn
  1. heart
  2. mind
  3. intention
  4. center
  5. core

classifiers: 顆, 個