What is the test for establishing a conspiracy?

"A conspiracy exists when one or more persons have the specific intent to agree or conspire to commit an offense, as well as the specific intent to commit the elements of that offense, together with proof of the commission of an overt act by one or more of the parties to such agreement in furtherance of the conspiracy. [Citations.] These facts may be established through the use of circumstantial evidence. [Citations.] They may also ' "be inferred from the conduct, relationship, interests, and activities of the alleged conspirators before and during the alleged conspiracy. [Citations.]" ' [Citations.]" (People v. Herrera (2000) 83 Cal.App.4th 46, 64.)