How does a back judge in H.S. know who is is his coverage man?

The BJ keys on the strong side of the formation or the LJ' side if both sides are equal. He normally keys on the outside receiver and the wingman keys the TE or a FL or RB coming out of the backfield. That's a very simplified explanation and there are probably exceptions, but that's basically it.

The NFHS Mechanic's manual has a new section this year that addresses keys. If you didn't get one, you need to.

In Texas we use CCA mechanics. Although the BJ keys there are very similar to what it sounds like they are in NF, one thing that needs to be pointed out is that a key is just important for the initial action of the play. As the play develops the BJ has the deepest receiver, regardless of where he came from. I trust it is the same in Fed?