Credible sources are written by authors respected in their fields of study. Responsible, credible authors will cite their sources so that you can check the accuracy of and support for what they've written.
This short video tutorial from the University of North Carolina, Chapel Hill Libraries discusses how the context in which you are going to use information is a critical part of evaluating potentially helpful information.
Remember when you're evaluating sources there are several factors that you need to consider when making a determination on whether it will work for you assignment or not.