Expert opinion should be discounted when their opinions could be predicted solely from information not relevant to the truth of the claims.
I can’t think of how to usefully determine that some sort of information is not relevant to the truth of the claims. In some sense, everything is; I can predict someone’s opinion on homeopathy by observing that they’re a doctor. Although you could say that being a doctor is relevant to the truth of the claims (people who choose to become doctors rather than homeopaths make this choice because medicine works and homeopathy doesn’t), it’s a rather indirect relevance.
I can’t think of how to usefully determine that some sort of information is not relevant to the truth of the claims. In some sense, everything is; I can predict someone’s opinion on homeopathy by observing that they’re a doctor. Although you could say that being a doctor is relevant to the truth of the claims (people who choose to become doctors rather than homeopaths make this choice because medicine works and homeopathy doesn’t), it’s a rather indirect relevance.