Ad verecundiam

An expert asserts A is true. Therefore A is true.

The expert, of course, may not be expert, but they are a touchstone that people use to avoid having their own expertise challenged.

You can also assert your own expertise. If the other person cannot challenge your credentials, then they cannot challenge your argument.

The dilemma with this appeal is not so much in the assertion of truth but in the true expertise of the so-called expert, who may be guessing or even joking. It is also known that if you bring together a group of experts then you are likely to get less than full agreement about any given question.

The expert may not be named (and is hence an anonymous authority) or may be absent and unable to answer probing questions. In this case, it is not known whether the person quoting the expert is quoting them accurately or even making the whole thing up.

Appeal to authority is a common method used in confidence tricks, where the confidence trickster sets themself up as an authority and so both dissuades the target from asking questions and encourages them to trust their 'expert' judgement.

However, in particular circumstances, it is sound to use as a practical although fallible way of obtaining information that can be considered generally likely to be correct if the authority is a real and pertinent intellectual authority and there is universal consensus about these statements in this field. This is specially the case when the revision of all the information and data 'from scratch' would impede advances in an investigation or education.

Further ways of validating a source include: evaluating the veracity of previous works by the author, their competence on the topic, their coherence, their conflicts of interest, etc.