What is the test for determining whether a defendant has a prior strike against the Three Strikes law?

As the United States Supreme Court stated: "The duty of this court, as of every other judicial tribunal, is to decide actual controversies by a judgment which can be carried into effect, and not to give opinions upon moot questions or abstract propositions, or to declare principles or rules of law which cannot affect the matter in issue in the case before it." (Mills v. Green (1895) 159 U.S. 651, 653.) Here, the trial court struck both prior strike allegations and thus the issues raised by defendant are moot. Accordingly, we do not consider defendant's challenges to the Three Strikes law.