Cite checking goes by many names, including Shepardizing, KeyCite, and authority checking, among others. At a basic level it is checking a case one plans to use to make sure it’s still okay. Rulings from cases can become invalid over time if a more recent case from the same or a higher court changes the rule, or if the legislature passed a statute that impacted the case. In order to find such events, legal publishers have created tools (called citators) to track such changes.
One of the original tools was Shepard’s (now a LexisNexis product). The online LexisNexis version allows a user to find documents that cite the case they are looking at. It also allows a user to see if any of those have overturned the case of interest, or otherwise challenged part of its ruling. In Shepard’s there are visual indicators to suggest a case is still good (green), called into question (yellow), or part of it has been overturned (red). Westlaw has a similar tool called KeyCite, and Fastcase uses Authority Check.
It is important to note that any of these tools can only indicate that there might be something. The user will have to read the newer case that may affect the original case to see what that impact actually is, and how it relates to the user’s situation.