If all the liver enzymes are elevated, Cushing's would not be my first guess. If only the alkaline phosphatase is elevated, then maybe, though there are other things it could be. This could be liver disease, could be tick disease, could be a number of things without further information.
Has this dog been on steroids for any length of time?
ketoconazole, a cheap antifungal, is often used in Cushing's, a trial might be warranted if they really think cushing's and have eliminated the other things.