In "Young Goodman Brown," what foreshadows Goodman Brown's meeting with the supernatural?

In Nathaniel Hawthorne's "Young Goodman Brown," there are several instances that point to Brown's plan to meet with the old gentleman who is actually the devil. (And Brown knows who he is meeting.)

The first bit of foreshadowing comes not from Faith—as he insinutates—but from Brown's words to her. As she begs him not to got out for the evening, and he gently chides her, asking if she does not trust him. She has said nothing to convey any mistrust of her new husband: he presents the topic.

My journey, as thou callest it, forth and back again, must needs be done 'twixt now and sunrise. What, my sweet, pretty wife, dost thou doubt me already, and we but three months married?

There is also foreshadowing in Faith's wish for him, as she sends Brown off on his "errand:"

And may you find all well when you come back.

Later we know that this will not be the case. There is also a sense of foreshadowing as Brown takes leave of his wife, offering words that sound almost like a spell against evil, or a child's prayer:

Say thy prayers, dear Faith, and go to bed at dusk, and no harm will come to thee.

As the young husband departs, he looks back as his wife who wears a sad countenance, and his commentary make one sure that he is doing something that he should not do—he infers that her goodness is such that to know what he is doing would do damage to her. He also makes a promise that after this one night he will do what is right and "follow her to heaven."

Poor little Faith!...What a wretch am I to leave her on such an errand! She talks of dreams, too. Methought as she spoke there was trouble in her face, as if a dream had warned her what work is to be done tonight. But no, no; 'twould kill her to think it. Well, she's a blessed angel on earth; and after this one night I'll cling to her skirts and follow her to heaven.

The setting described as Brown makes his way onward also foreshadows something dark and/or evil ahead, especially in that he has also resolved to be better in the days ahead (inferring he will not be "better" now)—that he has an "evil purpose."

With this excellent resolve for the future, Goodman Brown felt himself justified in making more haste on his present evil purpose. He had taken a dreary road, darkened by all the gloomiest trees of the forest, which barely stood aside to let the narrow path creep through, and closed immediately behind. It was all as lonely as could be...

So the reader gets the sense that Brown believes his wife is pure of heart and he is doing something he should not do. He utters words that sound like a protective spell—or perhaps a prayer. He infers that he will do this "thing" only one time, and never again. Then he walks a "dreary road," enters the woods (where Puritans believed the devil lived) which are dark and gloomy: where any evil could hide itself.

The fact that Brown meets the devil should be no surprise with the many examples of foreshadowing—they prepare the reader that there is some dark purpose to Brown's movements—and a dark person at the end of his "errand" is not unexpected.

In "Young Goodman Brown," what prompts the appearance of Goodman Brown's guide?

In addition to his concerns that "'the devil himself should be at [his] very elbow,'" Goodman Brown also makes plans for the future. He feels guilty for leaving his young wife, Faith (aptly and symbolically named), alone at home so that he can go on his errand. He believes that she may have had some premonition of the spiritual danger he would face, but then he thinks,

"[...] no, no! 't would kill her to think it. Well; she's a blessed angel on earth; and after this one night, I'll cling to her skirts and follow her to Heaven."

In other words, Brown thinks that he can set aside his Christian faith, symbolized by his wife, Faith, for now because it is somewhat inconvenient for him; he leaves it behind just as he leaves her. He obviously has some idea that his going into the woods on his "errand" is sinful and wrong, and that he ought not to do it, in part because it would "kill" her. However, this is not how faith is supposed to work: those with true faith try to live well and without sin all the time; they don't bargain and barter sinful nights for sinless days. It is shortly after this that the devil himself appears, as if to signal that Brown's plan will not be successful.

Your question refers to the thoughts of Goodman Brown as he takes his leave from his wife and beings to enter the dark and spooky forest, which is so crowded with trees that he reflects there could be any number of people out there hiding behind them, even though it appears Goodman Brown is completely by himself. This leads him to fear that others might be there with him who would do violence unto him, and he says:

"There may be a devilish Indian behind every tree," said Goodman Brown to himself; and he glanced fearfully behind him as he added, "What if the devil himself should be at my very elbow!"

Of course, this is very ironic, because the next moment he turns around a corner and finds a figure who we identify to be the "devil himself" waiting for him as he journeys on his way. Entering the spooky forest and the fear that it inspires in Goodman Brown thus triggers the arrival of the devil as teh father of all evil.

How do we know Brown has an appointment with a supernatural being in "Young Goodman Brown"?

For Hawthorne, the author of "Young Goodman Brown," the forest primeval is symbolic of a moral wilderness. In his story, Goodman ventures out on the night of the black mass; believing himself among the elect, he challenges the devil. For, Brown symbolizes man's predilection for evil as he is tempted to follow the path that leads to the forest.

When his wife Faith asks him to tarry with her, Goodman detects some trouble in her face,

...as if a dream had warned her what work is to be done tonight. 
With ...excellent resolve for the future, Goodman Brown felt himself justified in making more haste on his present evil purpose.

When Young Goodman Brown ventures forth, he wonders, "What if the devil himself should be at my very elbow!" Just then, a man approaches who bears

...a considerable resemblance to him, thouth perhaps more in expression than features.

Additionally, the old man has the appearance of one well acquainted with the world, and his staff resembles a "great black snake" and it even wriggles like a serpent.  And, there are other indications of the old man's evil nature:  He urges Goodman into the forest; he tells his companion that he is well acquainted with the Puritans; and he attests to having helped Brown's grandfather lash the Quaker woman through Salem's streets, and he helped Brown's father set fire to an Indian village. He has been with many other "select men" of "divers towns."