1. if it says .gov then I know it's a creditable source because not just anybody can get/edit a .gov site
2. If it is from a creditable or well known organization. If it's a big company known for giving accurate information then I know it's creditable
3. if the authors name and contact info is on the website. if the person contact info is there then it's highly likely creditable if the author is willing to give is info because he truly believe his info is acurate.
(it's like 12 AM so this is all I can t Hoi no of. Change the wording to however you want)