The criteria you would use to evaluate grey literature are the same as those used to evaluate any kind of information. Consider:
Evaluating Agency and Organization Web Sites
Although it is often suggested that searches for grey literature be restricted by domain, do not rely on the .org or .edu domains when evaluating Web sites. The .org domain no longer only applies to non-profits. Sites with URLs ending in .edu may also be personal student or faculty pages, rather than official university pages.
Also, don't count on the professional look of a Web page to indicate quality. Some very reputable sources of information can have Web sites that look dated. Some less reliable sources of information, on the other hand, are beautifully designed. Take a look at this site for RYT Hospital. It looks very professional until you look more closely at the content, which includes patient services such as "Head Transplantation".
If you are unfamiliar with an agency or organization, you can check to see if it is listed in reputable directories of organizations. Look it up in one of the directories of grey literature producing organizations listed on the Tips page.
The AACODS Checklist, put together by Jess Tyndal at Flinders University, is a good resource for more information on evaluating grey literature.
For an example of a good report, take a look at this one from Agency for Healthcare Research and Quality (AHRQ) Screening and Diagnosis of Gestational Diabetes Mellitus.