The expert should at least know about few examples of incorrect theories based on superficial similarities between languages. (S)he should be also informed about the number of coincidences arising from pure chance when two totally unrelated languages are compared. And (s)he knows miscellaneous facts which provide additional checks. E.g. you wouldn't believe a claim that English is ten thousand years old if you know something about the rate of language evolution or the history of germanic and indo-european languages.

To suppose that the experts don't know more than laymen seems weird. It may be true for some pseudo-scientific disciplines where is nothing to be actually known, but that's not the case of linguistics. I would yet admit that the experts are probably not less biased than laymen since the knowledge of biases is not usually taught.