Once you find cases that you intend to rely on, it is important to determine whether or not the legal propositions that they stand for are still "good law." Court opinions can lose their authoritativeness in a number of ways. A higher court can reverse a decision of a lower court in the same case, thereby undermining the precedential value of the lower court's opinion. Courts also sometimes overrule precedents or question the applicability of those precedents to the cases before them. Additionally, in California, a Court of Appeal opinion cannot be cited if the California Supreme Court orders it "not to be published" or "grants review" in the case (i.e., agrees to hear the case on appeal).
The major tool that is used by legal researchers to check the status of a case is called a case citator. Citators provide a view of the history of cases as they made their way through the courts. They also list subsequent cases and other authorities that have cited the earlier cases. Looking at a case's citation history can help you discover if there are any cases that have reversed, overruled, criticized, or otherwise negatively treated the cases you are interested in and intend to use. It can also help you find additional cases on the topic(s) in which you are interested.