lay one's finger on smth.

или put one's finger on smth.

   тoчнo oпpeдeлить, уcтaнoвить чтo-л.; cxвaтить cуть дeлa, пoпacть в тoчку

    You put your finger on the point with your usual excellent good sense (R. L. Stevenson). Sarah King, you see, had put her finger on the truth (A. Christie)