What is the test for "exigent circumstances" in the context of police actions?

In People v. Ramey (1976) 16 Cal.3d 263, 276, 127 Cal.Rptr. 629, 545 P.2d 1333, the court defined "exigent circumstances" as "an emergency situation requiring swift action to prevent imminent danger to life or serious damage to property, or to forestall the imminent escape of a suspect or destruction of evidence." The court recognized that "There is no ready litmus test for determining whether such circumstances exist, and in each case the claim of an extraordinary situation must be measured by the facts known to the officers." (Ibid. )