What a great question. I have wondered this myself before. Thank you for asking.
I sometimes have a hard time deciding what is a good resource and what is not. There is money to be made from both sides and I find it hard to trust either side. Some of the websites clearly have an agenda and that deters me from trusting them.
When I first began my research, I looked online, but it was overwhelming and I couldn't find many websites that I trusted. I do like the National Vaccine Information Center website. I trust that one more than others because their mission is not anti-vax, but pro-informed consent. So in short, the mission of the website plays a huge factor for me. Another criteria is the articles they use to back up their statements. I usually look at the actual articles (if I can). I look at whether it is a peer reviewed article or someone's opinion. I look at the sample size, who wrote it and other factors that may make the study unreliable.
Much of what I read, I won't repeat it to others as fact, I will just direct them to the site. My reasoning is to let them decide for themselves. Just because I decided something was reliable does not make it so.
As far as books go, I have found few books that I truly like on the subject. The one I always recommend is Romm's book. I know from a previous thread that you do not like Romm's books, but I found her book to be very unbiased. Even after reading it twice, I cannot decide if she is anti-vax or pro-vax. Her documentation is adequate, which is paramount. I want to be able to go to her sources and check what she is saying.
A book I read recently kept claiming that if you get Whooping Cough once you will have life-long immunity. I thought this was wrong so I googled it. I found several state health dept sites that stated that getting WC does not give you life-long immunity, although it may give you immunity for a few years. Because of this discrepancy, I was skeptical of everything else in the book. The author should research everything
she includes in her book.
I'm interested in how you decide what a reliable source is.