了如指掌（瞭如指掌）　

◎ 了如指掌 liǎorúzhǐzhǎng

[have sth.at one’s fingertips; know as one know his ten fingers; know like a book; know as well as a beggar knows his bag] 形容对情况极为清楚

对他们的活动,他完全了如指掌