What is the test for a prison officer to be found guilty of deliberate indifference by failing to prevent an assault?

demonstrate deliberate indifference to conditions posing a substantial risk or serious harm to an inmate; and it is well settled that deliberate indifference occurs when an official acted or failed to act despite his knowledge of a substantial risk of serious harm. Farmer, 511 U.S. at 834, 841; Clem, 566 F.3d at 1181; Hearns, 413 F.3d at 1040. The prison official need not "believe to a moral certainty that one inmate intends to attack another at a given place at a time certain before [he] is obligated to take steps to prevent such an assault." Berg v. Kincheloe, 794 F.2d 457, 459 (9th Cir.1986). However, before being required to act the prison official must have more than a "mere suspicion" that an attack will occur. See id.