The Library has many authoritative sources that are very helpful for fact-checking.
One example would be biographical dictionaries which list significant person's full names, birth dates, credentials, important achievements and often the names of all of their family members. Whether you need to confirm the name of the site of an archeological dig, or the date and details of a particular incident during the Bosnian war, the wealth of subject-specific encyclopedias can be very useful for confirming a wide variety of factual details.
This Guide lists only a very small sample of the Library's resources that may be useful for fact checking. The resource you need will be determined by the topic you're writing about.
The information under the Find Help tab provides suggestions on how to find help quickly.
This page from the CUNY Graduate School of Journalism, Research Center has an excellent outline of what types of information should be checked and has excellent tips and checklists on how to effectively fact-check information.
Factbooks provide current overviews or specific facts in a specific subject field. Here are 2 good examples:
The library has many language dictionaries and specialized dictionaries with subject focus. These are just a few examples that are available online.