You cannot judge how much an expert knows by what he chooses to tell you – only by what he answers to your questions. The first is under his control, the second is not.
Go to an expert with some questions for which you have researched the answers. If your future depends on his knowledge you are entitled to test it first.
Never go to an expert for advice without knowing more about your problem than he does. But it is more tactful to spare him the benefits of your instruction. Like the waiter he can always spit in your soup.
Never forget that doctors and lawyers used to use the tradesman’s entrance. Anyone to whom you pay a fee, directly or through taxation, is at your service. You have more interest in the health of your body or your rights than he has.
A professional is someone who uses particular skills for your benefit ahead of his personal interests. By that definition a plumber may be more professional than a lawyer.