What is the test for determining whether a prison official had the requisite knowledge of a substantial risk of harm?

For the current motion, "[w]hether a prison official had the requisite knowledge of a substantial risk is a question of fact subject to demonstration in the usual ways, including inference from circumstantial evidence, . . . . and a fact finder may conclude that a prison official knew of a substantial risk from the very fact that the risk was obvious." Id. (internal citation omitted); see also Lolli v. County of Orange, 351 F.3d 410, 421 (9th Cir. 2003) ("Much like recklessness in criminal law, deliberate indifference to medical needs may be shown by circumstantial evidence when the facts are sufficient to demonstrate that a defendant actually knew of a risk of harm.").