This is a tricky question because there is questionable stuff out there, but on the other hand a lot of scientifically supported therapies are probably working for reasons other than their purported reasons (due to publication bias, etc.). To complicate things more, there are some therapies that seem like quackery that seem to work for reasons no one really understands. So people purporting to be adhering to "empirically supported therapy" (a kind of political buzzword) might actually be doing no better than someone else who doesn't explicitly advertise that, but who is rigorously and critically evaluating the scientific literature.
My advice is to pay attention to where someone got their degree, and how they describe themselves. By degree I don't mean you need to fetishize invalid status stuff, but look for people from degree programs you trust. If you don't feel comfortable with someone, look elsewhere. Don't feel uncomfortable asking them about their theoretical orientation or thinking about cases.