What is the test for establishing that a plaintiff has a legal interest in a property in controversy?

'Such an action is brought, as authorized by the statute, 'for the purpose of determining' any adverse claim that may be asserted therein by a defendant to the land in controversy; and this does not mean that the court is simply to ascertain, as against a plaintiff shown to have a legal interest, whether or not such defendant has Some interest, but also that the court shall declare and define the interest held by the defendant, if any, so that the plaintiff may have a decree finally adjudicating the extent of his own interest in the property in controversy. The object of the action is to finally settle and determine, as between the parties, all conflicting claims to the property in controversy, and to decree to each such interest or estate therein as he may be entitled to. Of course, if the plaintiff fails to show any legal interest in the property in controversy, and as to which he asserts title, he must fail altogether, and could not complain of a judgment of nonsuit, but where he shows any legal interest he is entitled to have that interest declared by the court. (Citations omitted.) By the order granting a nonsuit the plaintiff was, in effect, declared to have no legal interest whatever in the property in controversy. This was, of course, erroneous.' (Peterson v. Gibbs, Supra, 147 Cal. at pp. 4--6, 81 P. at p. 122; emphasis in the original).