What is the test for proving an official was subjectively aware of a risk to inmate health or safety?

In order to prove that an official is subjectively aware of a risk to inmate health or safety, a plaintiff inmate need not produce direct evidence of the official's knowledge; a plaintiff can rely on circumstantial evidence indicating that the official must have known about the risk. See Hope v. Pelzer, 536 U.S. 730, 738 (2002) ("We may infer the existence of this subjective state of mind from the fact that the risk of harm is obvious"); Wallis v. Baldwin, 70 F.3d 1074, 1077 (9th Cir. 1995).