Probing is a well-known admission control technique that can achieve high utilization and per-flow quality of service in a scalable way. We have recently introduced an extension to the basic probing technique, called ε-probing, to overcome a resource stealing problem that impairs the use of probing in systems with multiple service classes. In this paper we describe an experimental system that was designed to evaluate the effectiveness of both probing and ε-probing techniques. We have developed a software module that implements the probing functionality, which can be inserted in end hosts or edge routers. Several tests were carried out to study the effect of various system parameters in the performance of the probing techniques. The results clearly show that both probing techniques are able to accurately perform admission control while achieving high utilization. Moreover, they also show that in environments with multiple service classes such as DiffServ, ε-probing can eliminate the resource stealing problem, providing an effective solution to support per flow QoS without signaling and without maintaining flow state at core routers.