In "The Devil and Tom Walker," how does Tom know the man he meets in the forest is the devil?

Irving offers several clues in his description of the man in the forest so that Tom and readers know that he is the devil.

1. The man seemingly lives in the forest when Tom meets him.  Puritans from the story's setting believed that the devil roamed the woods around their communities, waiting to tempt humans into signing his black book. The devil even ask Tom what he is doing on "his grounds," revealing that the dark forest is his domain.

2. He is called "the black man," but Irving is not describing his race with that description.  He goes on to write:

"His face was neither black nor copper colour, but swarthy and dingy and begrimed with soot."

The soot suggests that this creature spends most of his time near fire (hell-fire).

3. When the devil and Tom are finished talking, he presses his thumb against Tom's forehead, and Tom cannot remove the black, sooty mark when he gets home.

4. Finally, the devil is in the process of burning trees (which represent people's souls) when Tom comes upon him. Only a supernatural creature like the devil would have such power.