The Kav Hayashar, Chapter 52, relates stories about people who made themselves look extremely religious, though were really very low individuals. One story is about a man who would sit in Shul all day, wrapped in his talis and tefilin, learning and davening. He seemed like a real tzadik, but it turned out that he and his wife were great sinners; they even ate chometz on Pesach and they ate on Yom Kippur. In the end they converted out of the faith. He also relates a story about a woman that made herself out to be so modest, even telling her husband she didn't want to go out of the house, for fear of enticing men. She turned out to be an adulterer, and was beheaded by the king.
He ends off the chapter saying that from these stories we see that one shouldn't rely on what he sees with his eyes. (Don't judge a book by its cover.) If you want to know who is definitely a upright Tzadik, see who deals in business honestly and with emuna. If you see someone who "acts" very religious, but doesn't conduct himself properly in business, then distance yourself from him in every possible way, because the main yirah and tzidkus is in money dealings. He then states a second time that one who acts righteously in monetary matters is a "Tzadik Gamur".