A simple way is to apply the CRAAP guidelines.
Currency - When was the material published? On websites look for copyright information or the last date it was updated.
Relevance - How does the source pertain to your research? Who is the intended audience? Make sure that your source is directly related to your topic or thesis and that the writing is appropriate for your project.
Authority - Who wrote or published the work? Does the work or website provide the authors credentials? If you're working online, take a look at the end of the url, it can reveal a lot about the organization (e.g. .gov = government, .edu = school/university). If you cannot tell who published or created a work that is a red flag that it is not a good source.
Accuracy - Where or how did they gather these facts? Can you verify their information in another source? Do they provide a bibliography or citations for any claims that they have made? If you're looking a journal article, is it a peer-reviewed article? Take a look at the general layout of the work - if there are a lot of typos or mistakes that is a sign that it is not accurate.
Purpose - Why have they written about this topic? Are the ideas based on legitimate research or is it an expression of an opinion or a rant/exaggeration/parody? Are they trying to inform, entertain, or advertise to you?
Need more help? Contact our reference desk: 540-362-7465, email us at firstname.lastname@example.org or make an appointment to discuss your research with a librarian.