What is the test for determining whether a plaintiff is a permanent employee or a temporary employee of a company that has acquired a property interest sufficient for a hearing?

In the case at bar, the defendants contend that the plaintiffs are employees at will not entitled to a hearing. The plaintiffs contend that they are permanent employees who have acquired a property interest sufficient to vest them with the due process right to a hearing. The determination of the nature of the plaintiffs' interests must be made in accordance with state law. Id. at 577, 92 S.Ct. at 2709, Bishop v. Wood, 426 U.S. 341, 344, 96 S.Ct. 2074, 2077, 48 L.Ed.2d 684 (1976).