Criticism of the sources refers to the attempt to establish the sources used by the author and/or redactor of a final text.
You must be able to critically scrutinize and evaluate the information you are about to use in your pieces of work. This implies identifying the source and its authenticity, defining its value over time and space, establishing the objectivity or bias of the source, or its relation to similar sources within the subject.
Here is a movie from SLU about source criticism on the Internet.
Source criticism - searching the Internet
A movie from SLU.