Which online source can be used to check if a case is still valid law?

To determine whether a case remains valid law, all the listed options are reliable and widely used tools in legal research that serve this purpose.

LexCite is a useful resource that provides citation information and analysis, helping researchers identify how a case has been cited in subsequent decisions. Similarly, KeyCite is an essential feature of Westlaw, allowing users to check the current status of a case, including whether it has been overruled or adversely treated by later cases. Shepard's Citation Services, found within LexisNexis, serves the same purpose by providing citation history and indicating the validity of case law.

Each of these services contributes importantly to legal research by ensuring that practitioners and scholars are referencing cases that are still applicable and reliable, highlighting how cases interact with each other in terms of validity. Therefore, the correct response encompasses all options listed as they collectively enable legal professionals to verify the current status of case law efficiently.