了：了解；清楚。指掌：指手掌里的东西。形容对情况清楚得就像指点掌上的东西；给人看一样。比喻对事物了解得非常清楚。

先秦 孔子《论语 八佾》：“或问禘之说。子曰：‘不知也；知其说者之于天下也，其如示诸斯乎！’指其掌。”

只这一相形之下，美丑高低，便了如指掌了。（闻一多《冬夜评论》）

一目了然、一清二楚、洞若观火

失手；熟手；空手

know something like the palm of one's hand <know as one know his ten fingers; know as well as a beggar knows his bag>

掌(たなごころ)を指す