What is the ultimate test of possession of a prohibited substance?

"It is not scientific measurement and detection which is the ultimate test of the known possession of a narcotic, but rather the awareness of the defendant of the presence of the narcotic. Guilt or innocence on a charge of illegal possession may not be determined solely by the skill of the forensic chemist in isolating a trace of the prohibited narcotic in articles possessed by the defendant. As forensic science, measuring devices and techniques improve, smaller and smaller amounts of residue are required for the chemist to detect the presence of the narcotic. The presence of the narcotic must be reflected in such form as reasonably imputes knowledge to the defendant." (People v. Aguilar (1963) 223 Cal.App.2d 119, 122-123, 35 Cal.Rptr. 516, italics added, quoted in People v. Leal, supra, 64 Cal.2d at p. 509, 50 Cal.Rptr. 777, 413 P.2d 665.)