What is the test for a detention under the Fourth Amendment?

manifestation' that criminal activity is afoot and that the person to be stopped is engaged in that activity." (People v. Souza (1994) 9 Cal.4th 224, 230.) "A detention is reasonable under the Fourth Amendment when the detaining officer can point to specific articulable facts that, considered in light of the totality of the circumstances, provide some objective manifestation that the person detained may be involved in criminal activity." (Id. at p. 231.) "'The possibility of an innocent explanation does not deprive the officer of the capacity to entertain a reasonable suspicion of criminal conduct. Indeed, the principal function of [police] investigation is to resolve that very ambiguity and establish whether the activity is in fact legal or illegal . . . .'" (Id. p. 233.)