Grey literature should be appraised and evaluated:
Authority: Who is the author/source? What are their qualifications? Are they an expert in the area?
Objectivity: Is there bias? How are the conclusions justified? Does the literature promote a product/service?
Intended Audience: Who is the source aimed at? Is it the general public or scientific community? Has appropriate academic rigor be applied?
Accuracy: Are the facts/figures, dates cited, and quality of evidence reliable and valid? Is the information cited and references included?
Currency: How old is the information? When was it created? Has it been superseded by more recent information available?
Try using these checklists:
Watch this two minute video from UTS Library on the C.R.A.P test to evaluate resources.