Over the years I've learned that most of the time people tell you they are going to rip you off before hand. Not flat out directly, but they'll provide deliberate warnings because it makes it makes it easier to rationalize what they are going to do. The best test for whether or not someone will pay me is whether or not they will write me a check up front. People who intend to pay are ok with it, people who don't intend to pay aren't.