This question struck me as I read a piece in the American Scholar, Vol. 75, Spring 2006 at 125 (Nan Stone). Stone gave no advice, but it is worthwhile to fall back on native intelligence for some answers, since many general counsel wrestle with this situation.
Ask intelligent questions. Ask for a translation of jargon. Step back from the perspective of the subject matter expert. Evaluate the assumptions and traditions that frame the analysis. Force the guru to articulate knowledge, premises, and reasoning. Pursue a few what-if scenarios.