Often they'll use coercive methods to bring you into their orbit and cult-like techniques to keep you there. Some may even threaten to reveal privileged information about you if you leave. Others may claim that the patient will surely regress, and the blame will all be on you. Money can be another red-flag issue. Doctors and therapists deserve to get paid, but the cost of treatment should be in proportion to the cost of similar therapies.
Most reputable therapists are in the field of treating patients with PDDs because they care.
They want to make an adequate living, but they aren't expecting to live a lavish lifestyle on other people's misery. Ask about costs. If the answer sounds like mumbo-jumbo instead of a truthful accounting of expenses, find the door. Be especially careful of any method that claims to cure PDDs. People do get better with proper intervention.