Check out a practitioner's credentials. Letters after a name can stand for a lot of things, not all of them useful. Learn the buzzwords used in a particular therapy, so you can sound knowledgeable when you ask questions. People who do have something genuinely new to offer are usually happy to attempt a layperson's-level explanation. If you're told it's too complex or mysterious to explain, a red flag should go up in your mind.
Nothing is ever so new that it has no relationship to what has gone before.
Ask the practitioner to contrast his approach to mainstream methods, such as ABA or sensory integration. Dr. Lovaas is widely admired by grateful families, and rightly so, but no one hangs on his every word. He's a good doctor, a good researcher, and an important guy in the field of autism, not a guru. That's as it should be. Watch out for therapies that seem to be hitched to a cult of personality.