What is the test for establishing probable cause to arrest based on information obtained through the use of an informant?

Probable cause to arrest may be based in whole or in part upon information obtained through the use of an informant, in which case the court must "assess the information by examining the 'totality of the circumstances' bearing upon its reliability." United States v. Smith, 9 F.3d 1007, 1012 (2d Cir. 1993) (citation omitted). "[I]t is established in this circuit that evidence sufficient to show probable cause by corroborating even a previously unknown informant may be found in circumstances which do not actually establish the crime itself." United States v. Rueda, 549 F.2d 865, 870 (2d Cir. 1977). "An untested informant's story may be corroborated by other facts that become known to the [arresting agent], even if they corroborate only innocent aspects of the story." United States v. Sultan, 463 F.2d 1066, 1069 (2d Cir. 1972).