What is the test for establishing a conspiracy?

Conspiracies are often proven through inference. In Paradis v. The King, 1933 CanLII 75 (SCC), [1934] S.C.R. 165 at 168 Rinfret J., as he then was, said that: . . . Conspiracy, like all other crimes, may be established by inference from the conduct of the parties. No doubt the agreement between them is the gist of the offence, but only in very rare cases will it be possible to prove it by direct evidence. Ordinarily the evidence must proceed by steps. The actual agreement must be gathered from “several isolated doings”, (Kenny - “Outlines of Criminal Law”, p. 294) having possibly little or no value taken by themselves, but the bearing of which one upon the other must be interpreted; and their cumulative effect, properly estimated in the light of all surrounding circumstances, may raise a presumption of concerted purpose entitling the jury to find the existence of the unlawful agreement.