What is the test for a claim that substantial evidence does not support that a defendant possessed a usable quantity of heroin?

We think this case is actually most similar to People v. Stafford (1972) 28 Cal.App.3d 405, in which the reviewing court rejected a contention that substantial evidence did not support the defendant's possession of a usable quantity of heroin. (Id. at p. 413.) The court explained: "[The testifying chemist] did not . . . weigh the capsules or their powdery contents, and he did not testify to any opinion as to the quantity of powder or heroin in the capsules. Nor does the written record contain any evidence concerning the quantity of the contents of the capsules. We have, however, inspected the exhibit. The capsules are transparent. They are full of white powder. We hold that the real evidence, together with the chemist's testimony, creates a prima facie case that defendant possessed a usable quantity. [Citation.]" (Id. at pp. 413-414.)