Many small pieces of information can help you assess an author's credibility for an annotated bibliography. Look or search for the author's credentials/affiliations (i.e., university professor? where? in what field or department--is it one relevant to the article?, etc), previous work (have they written on this topic before?), or any other biographical information you can find. You might need to do a separate Google search to find this type of information, or if you're working in a database, you might find what you need in the article record (the page with the abstract, title, page numbers, etc).
If you are trying to evaluate a source where the author is not listed by name or you can't find information about the individual writer, you can also consider the authority/credibility of the organization publishing the information.
