What is the test for treating a corporation as an entity separate and distinct from its stockholders?

"'It is the law in California as elsewhere that, although a corporation is usually regarded as an entity separate and distinct from its stockholders, both law and equity will, when necessary to circumvent fraud, protect the rights of third persons and accomplish justice, disregard this distinct existence and treat them as identical.' The issue is not so much whether, for all purposes, the corporation is the 'alter ego' of its stockholders or officers, nor whether the very purpose of the organization of the corporation was to defraud the individual who is now in court complaining, as it is an issue of whether in the particular case presented and for the purposes of such case justice and equity can best be accomplished and fraud and unfairness defeated by a disregard of the distinct entity of the corporate form." (Kohn v. Kohn (1950) 95 Cal.App.2d 708, 718.)