When evaluating sources of free health information online, first try to learn more about the organization producing the information, and determine what their agenda is. Look for an "About" page on the website to determine the name of the organization producing the site.
Government resources can be trusted before sites that appear to have a commercial/profit motive, so always look for information from the US Centers for Disease Control and Prevention (CDC), Food and Drug Administration (FDA), National Institute of Health (NIH), National Library of Medicine (NLM), National Center for Biotechnology Information (NCBI) or the World Health Organization (WHO), before trusting unknown sites.
To vet other sites, look carefully at how they get their data, how often they update it, and whether it appears to be a widely-used site. It's also a good idea to check for Wikipedia articles about the site and the organization that produces it, to see if they are notable, and what people are saying about their credibility. You can also try applying the CRAAP test. When in doubt, as a medical librarian for help!
There are some resources you can find online that provide medical information in exchange for hosting advertisements that provide them with revenue. These commercial sites may have accurate information, but government sources should be preferred where possible.