Nope, just this comic about a cornerwitch, which doesn't cover asking bricks questions and is more about the kind of objects one might Deal for: http://charminglyantiquated.tumblr.com/post/113043123123/a-short-comic-about-witches-and-wishes-and-wanting If you want to keep it simple, what it says on the tin: simply talking to bricks or concrete and looking at graffiti.

I love the idea of its existence being in question, but that may make accessibility to a Plot Role more difficult, so try to keep "office hours" vague, e.g. it "sometimes" vanishes. It also sounds more like Elsewhere influence than something within the cornerwitch's power, so he may simply have taken advantage of a Thin Spot.