In "Seventy-Six Dollars," how does Monk convince Angel Carter he has superpowers?

Monk starts by guessing the city where Angel's grandmother lives, Macon, Georgia. While at first she does not seem to think the exact city, he gets her to tell someone else the answer, as proof in case he gets it right. When she tells the boy, he can hear her say the city in his mind. Afterwards, Monk doubles the bet and lets Angel set the question. She asks him to tell her how much her hairdo the day before had cost her.