Regurgitation and vomiting are usually distinguishable by the head movements that accompany them. Generally, regurgitating is accompanied by bobbing the head straight up and down and other attention seeking behaviours like perhaps quivering wings for example. The regurgitated food should not be foul smelling, and they might even eat or swallow it again. With vomiting they usually shake their head from side to side more violently and the vomit will be flung around and perhaps end up landing on the bird's head and body. They want to get that stuff as far away from them as they can so it can be flung, sometimes quite some distance, by the slinging motion of the bird's head. It may be foul smelling too.
