Credibility refers to whether something is believable and trustworthy. When you look at Internet information, you want to know the basis of the author's authority. You need to know how to look for information and clues to decide whether the author or the writer is really qualified to say the things he/she is saying. Assessing credibility means that you have to examine the sources of the information i.e.

1. Who wrote the information?
2. Who are they (whether individuals, groups, or institutions)?
3. What experience or qualifications do they hold relative to the material they are providing?
4. What interests do they have in promoting certain information and points of view ?

Once you have found the names of the authors, look to see if there is some way of contacting them. This is usually found at the bottom of the page. There should be some kind of contact information.

The authority of the author is often related to the legitimacy of the sponsoring organization. A credible sponsoring organisation is not likely to host information by non-credible authors. Look at the web site address and the logo on the page to see if you can establish something about the organisation that hosts the site. For example if the web site ends in edu, it is likely to be a university, but if the web site has k12 or school in it, this is a school. Government sites often have gov in the web site address. 

The use of a ~ in a web site address e.g. http://www.linfield.edu/~pswanso/  (online) indicates that the information appears on a personal web page. This normally means that the information on that page (Peter Swanson's site in this case) may not carry the same credibility as the sponsoring organization (which is an edu address in this case - an educational institution).