What is the test for using an expert opinion to prove a defendant had the capacity to commit a crime?

"Also, an expert may not offer an opinion regarding whether the defendant had the capacity to form the intent required for the crime, or whether the defendant actually did form the requisite intent." (People v. Smithey, supra, 20 Cal.4th at pp. 960961, 86 Cal.Rptr.2d 243, 978 P.2d 1171 ; People v. Samayoa (1997) 15 Cal.4th 795, 835837, 64 Cal.Rptr.2d 400, 938 P.2d 2.) However, where gang activity is relevant to a case, the use of expert opinion testimony to prove various aspects of gang activity, including those which may be highly probative of the defendant's intent, is well-established. (People v. Gonzalez , supra , 38 Cal.4th at p. 946, 44 Cal.Rptr.3d 237, 135 P.3d 649.)