了如指掌 (瞭如指掌) liǎo rú zhǐ zhǎng

liǎo rú zhǐ zhǎng set phrase to understand as [clearly as] pointing to the palm of the hand
Domain: Idiom 成语
Notes: (CC-CEDICT '瞭如指掌'; Guoyu '瞭如指掌')