What is the test for a conspiracy to commit a crime?

"'Conspiracy requires two or more persons agreeing to commit a crime, along with the commission of an overt act, by at least one of these parties, in furtherance of the conspiracy.' [Citation.]" (People v. Homick (2012) 55 Cal.4th 816, 870.) Conspiracy requires the intent to agree, and the intent to commit the underlying substantive crime. An agreement among alleged conspirators is often established by circumstantial evidence. Thus, the existence of a conspiracy may be inferred from the conduct, relationship, interests, and activities of the alleged conspirators before and during the alleged conspiracy. Common gang membership may be part of the circumstantial evidence that supports the inference that perpetrators acted as conspirators. (People v. Superior Court (Quinteros) (1993) 13 Cal.App.4th 12, 20.)