Current literature defines the effective capacity of a wireless link as the maximum throughput that can be supported while meeting specific Quality of Service targets on packet delay. This metric can be harnessed for a wide variety of QoS control routines within wireless networks such as traffic optimisation, delay sensitive admission control and routing. However to date, no measurement tool has been proposed to estimate the effective capacity. In this article we present CrEST, an effective capacity estimation tool for wireless networks. CrEST is an active based measurement tool reliant on the effective capacity model. To validate the tool we carry out a sensitivity analysis of the tools configuration parameters and identify how these choices impact on the tool performance in respect to convergence time and overhead incurred. We find that CrEST, if appropriately configured, can deliver accurate effective capacity estimations suited to a broad range of measurement scenarios.