Valley of Fear, Act 1: What was the secret code of the Scowrers to recognize each other?

Scrowers have a special hand shake in addition to a hand signal in order to recognize each other.

A hand-grip passed between the two.

“I see you speak the truth,” said the workman. “But it’s well to make certain.” He raised his right hand to his right eyebrow. The traveller at once raised his left hand to his left eyebrow.