two most important aspects are site's purpose and the original source of the information provided. These two tells a lot about the credibility of the website, if the original source is scientific based, and if the purpose is further more marketing, then that health website is trustworthy.
These aspects are important, because in the Internet there are too much scams, and you have to develop a filter to only feed from those trustworthy websites, even more if it's about your health.