How do they know it was Mr. Hyde? Chapter 2

Utterson waited for Hyde until he finally ran into him as he arrived at Jekyll's home. Hyde answered to his name, and thus, Utterson knew he had found the elusive Hyde.

Mr. Utterson stepped out and touched him on the shoulder as he passed." Mr. Hyde, I think?"
Mr. Hyde shrank back with a hissing intake of the breath. But his fear was only momentary; and though he did not look the lawyer in the face, he answered coolly enough: "That is my name. What do you want?"