When Tom first meets him, the devil is described as "a great black man" who is "neither Negro or Indian," thus making his appearance immediately unusal. The devil's face is described with these details:
. . . his face was neither black nor copper-color, but swarthy and dingy, and begrimed with soot, as if he had been accustomed to toil among fires and forges.
Another detail follows immediately that suggests the figure Tom sees is a supernatural being: he has "a pair of great red eyes."
To answer this question, take a look at the description of the devil when he makes his first appearance in the text. This occurs early in the story when Tom finds an Indian skull in the woods which he kicks in temper. The devil then makes his appearance and, as Tom surveys him, he notices that the devil is neither "negro" nor "Indian." In fact, he has a "swarthy" complexion and is "begrimed with soot." This leads Tom to makes an important observation: that the devil looks as though he is "accustomed to toil among fires and forges." In other words, that the devil comes from a place of hellfire.
In addition, Tom notes that the devil has "red eyes," a color which is synonymous with hell and fire.
Finally, towards the end of their meeting, the devil provides Tom with his signature by pressing his finger onto Tom's head. Later, Tom notices that this mark cannot be removed by any means; his skin has literally been burned by the devil's finger. This act of burning further reinforces the notion that the devil has come from a place of hellfire.