了若指掌 (瞭若指掌) liǎo ruò zhǐ zhǎng

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