Snopes is probably the most trusted online fact-checking website. It has been verifying the accuracy online information since the dawn of the Internet.
Politifact is a well-known fact-checker that specializes in determining the accuracy of politicians' statements and arguments.
Factcheck.org is another trusted non-profit, non-partisan fact-checking organization.
Whois can help you determine who registered a particular domain. This can be helpful when you want to find out who owns a website.
FirstDraftNews Fact-checking Course
If you are interested in going a little more in-depth into how fact-checkers verify sources, you can take a free, online course developed by FirstDraftNews at the Harvard Kennedy School of Government. Sign-up for the course here.