Use the following to help establish a baseline of what is a "credible" source.
If it is a web page:
Is the web page as good as (or better than) what I could find in journal/magazine articles or other published literature that is not on the free, general web?
If it is an article:
Is the article as good as (or better than) other articles I may be able to find?
Do I need to change my search terms?
Do I need general or a specific subject database?