“I work as an autism specialist and also in behavior modification techniques. I am not a licensed professional but have many years of experience. If I give advice to someone over the phone or in person about non- physical behavior modification techniques that they will use with their own child, what level of liability could I potentially be held at? They do pay me for this service.”
This depends in large part on both the state in which you are practicing, and in what area of specialty you would be considered to be practicing. Anybody can, of course, sue anybody for anything – the real question is, would they win? Your best bet would be to contact whatever governing body exists for people who work in your field but are licensed, if such a thing exists.
Additionally, if this is an area in which people with medical backgrounds or licenses also practice, I might be at least as, if not more, concerned about the overseeing medical association suing you for practicing without a license.