Proverb meaning that if you scratch beneath the surface you'll find faults or dirt — look closely enough and you'll find something bad.
あの会社は表向きは立派だが、叩けば埃が出ると言われている。
That company looks respectable on the surface, but people say that if you investigate you'll find dirt.
彼は完璧に見えるが、叩けば埃が出るタイプだよ。慎重に付き合ったほうがいい。
He seems perfect, but he's the type where flaws show if you look — better to be cautious in dealing with him.
当時の政治家に関しては、叩けば埃が出ることが次々と明らかになった。
Regarding the politicians of the time, it became clear one after another that dirt would be revealed upon investigation.