Until now, we have primarily been focused on how to find books and journal articles through the Library's resources. Ideally, everything you find through the library's tools would be legit. Unfortunately that is not always the case. Whether you have identified a resource through Google, or a scholarly index you must critically evaluate every source before you use it.
Iowa State University's library have developed a checklist for evaluating scholarly books and articles. To summarize, the following elements should be examined: (1) author, (2) publisher, (3) peer review, (4) purpose, (5) content, (6) usefulness, (7) accuracy, and (8) currency. My advice: trust your gut!
There are also a number of decent checklists that have been developed to help you evaluate websites. One of these checklists, the CRAAP Test, was developed by Meriam Library (California State University, Chico). It delivers a list of questions to help you evaluate the information you find.
For a useful CRAAP worksheet, refer to this rubric attributed to Central Library MCHS. It prompts you to score a site on each of the CRAAP criteria to come up with a final score that could range between a potentially "highly questionable source" to a "excellent source for research."