Spasticity is a serious problem that affects a large percentage of patients affected by neurological disorders including stroke, multiple sclerosis, cerebral palsy, traumatic injury. Unfortunately, no reliable assessment method of spasticity is currently available. This is a critical missing tool for a rational, assessment-based approach to rehabilitation of patients with neurological impairment. In this paper we propose a robot-based assessment method, focused on the wrist joint, that could be readily integrated with robot treatment protocols. The method consists of the delivery of small and quick rotational disturbances to one of the Degrees of Freedom (DoF) of the wrist (Flexion/Extension DoF). With the aim of validating its consistency and reliability, the method was preliminary tested on a population of healthy subjects, providing a promising initial operational basis.