What is the test for proving the existence of a criminal street gang?

"In reviewing a challenge to the sufficiency of evidence, 'the reviewing court must consider the evidence in a light most favorable to the judgment and presume the existence of every fact the trier could reasonably deduce from the evidence in support of the judgment. The test is whether substantial evidence supports the decision, not whether the evidence proves guilt beyond a reasonable doubt.' [Citation.] We consider whether a rational trier of fact could have found the essential elements of the crime beyond a reasonable doubt. [Citations.] To prove a gang allegation, an expert witness may testify about criminal street gangs." (People v. Romero (2006) 140 Cal.App.4th 15, 18.)