Available bandwidth - as well as capacity or achievable bandwidth - on a path or a link is one of the very important parameters to measure or estimate in a network: it is of high interest for many networking functions (routing, admission and congestion control, load balancing, etc). Active probing techniques provide the easiest and the more flexible approach, for estimating available bandwidth. In addition, they can be used for different network technologies or structures. Many techniques and tools for available bandwidth estimation appeared recently, but little attention has been given to the accuracy of the estimated values in the real Internet, most of previous studies focusing on validating the accuracy of these tools on local platform. Therefore, this paper deals with evaluating the accuracy of active estimation tools in the real wide area Internet. We use passive monitoring tools for this purpose. We then built a platform combining active and passive equipments, and define a methodology for evaluating active probing techniques using passive tools. The passive evaluation relies on DAG system cards that represent references for such kind of measurements. This paper then discusses the results we got in the different experiments with different tools. In particular, we use traffic generators for changing the characteristics of the traffic on the Internet paths, which we are making our measurements on. It is useful for analyzing the accuracy of active estimation tools according to network and traffic conditions.