Decisions of courts, or case law, establish precedent. Case law is one source of legal authority, along with statutes and regulations. Before you rely on a case, be sure to update case law to be sure another case has not overruled it.
To find relevant case law on a topic, start with a secondary source, and mine the footnotes for citations to other sources, including cases, on your topic, Once you have looked at secondary sources, you can also search case law.
If a more recent case questions or even overrules the case you located, citators (Keycite in WestlawNext or Shepard's in Lexis Uni) will let you know. Citators also serve as a research tool - identifying additional cases on your topic.