In this paper we define a probabilistic testing semantic:; which can be used to alternatively characterize fair testing. The key idea is to define a probabilistic semantics in such a way that two non-probabilistic processes are fair equivalent iff any probabilistic version of both processes are equivalent in our probabilistic testing semantics. In order to get this result we define a simple probabilistic must semantics by saying that a probabilistic process must pass a test iff the probability with which the process passes the test equals 1. Finally, we present an algorithm for deciding whether the probability with which a finite-state process passes a finite-state test equals 1. Alternatively, this algorithm can be used for computing whether a finite-state process fairly passes a finite-state test.