瞭若指掌

see 了如指掌[liao3 ru2 zhi3 zhang3]

(of eyes) bright / clear-sighted / to understand clearly

unofficial variant of 瞭[liao4]

to watch from a height or distance / to survey

to watch from a height or distance

to seem / like / as / if

finger / to point at or to / to indicate or refer to / to depend on / to count on / (of hair) to stand on end

palm of the hand / sole of the foot / paw / horseshoe / to slap / to hold in one's hand / to wield