What is the test for establishing that there is a conspiracy?

In addition, we have recognized that " '[a] conspiracy by its very nature is a secretive operation.' " Id. (quoting United States v. Provenzano, 615 F.2d 37, 45 (2d Cir.), cert. denied, 446 U.S. 953, 100 S.Ct. 2921, 64 L.Ed.2d 810 (1980)). Thus, we have held that the existence of "a conspiracy ... may be established ... through circumstantial evidence." United States v. Sanzo, 673 F.2d 64, 69 (2d Cir.), cert. denied, 459 U.S. 858, 103 S.Ct. 128, 74 L.Ed.2d 111 (1982). Moreover, to be sufficient "the evidence need not have excluded every possible hypothesis of innocence." United States v. Soto, 716 F.2d 989, 993 (2d Cir.1983).