What is the test for establishing a criminal conspiracy?

'particularly when those circumstances are the defendant's carrying out the agreed-upon crime.' [Citations.] To prove an agreement, it is not necessary to establish the parties met and expressly agreed; rather, 'a criminal conspiracy may be shown by direct or circumstantial evidence that the parties positively or tacitly came to a mutual understanding to accomplish the act and unlawful design.' [Citation.] [] . . . Because there rarely is direct evidence of a defendant's intent, '[s]uch intent must usually be derived from all the circumstances of the attempt, including the defendant's actions.' [Citation.]" (People v. Vu (2006) 143 Cal.App.4th 1009, 1024-1025 (Vu).)