Why does this person have any business telling me about this subject? What makes her or him an expert?
Check her or his educational background. Does s/he have an advanced degree in THIS field from a reputable institution?
What is his/her occupation? Academia? Think tank? Government researcher?
Has s/he published about this before?
Educational information and publication history may be included in the source (on the back cover of a book, for example), but if not, you can check the Biography in Context database for their biographical information. You could, of course, also Google them. But keep in mind that the information you find there might be self-reported (or self-fabricated).
If you want to dig deeper, you can find out if others have cited her/his work in their own scholarship by searching Web of Science.
What do you know about the organization publishing or hosting the source?
Occupation: Actress/Model/Autism Activist (former Playboy Playmate of the Year)
Publications: 7 books, including A Mother's Journey in Healing Autism
Dr. Elizabeth Miller
Occupation: Head of Immunisation Dept, Communicable Disease Surveillance Centre, London UK
Publications: 16 Journal Articles, including "Autism and measles, mumps, and rubella vaccine: no epidemiological evidence for a causal association" from the Lancet
Easy, right? But sometimes you have to dig deeper. Compare Dr. Miller (above) with Dr. Wakefield:
Dr. Andrew Wakefield
Occupation: Medical Researcher
Publications: Over 200 journal articles, including "Autism, viral infection and measles-mumps-rubella vaccination"
|On the surface, he looks very reputable. Now look him up in Biography in Context.|
Jenny McCarthy: John Shearer / WireImage / Getty Images / Universal Images Group (via ImageQuest)
Elizabeth Miller: http://news.bbc.co.uk/2/hi/programmes/panorama/1795534.stm
Andrew Wakefield: http://www.globalresearch.ca/wp-content/uploads/2015/03/andrew-wakefield-600-apr-12.jpg