A new circuit-based method for performance evaluation of mid-range wireless power transfer links is presented. The characterization is based on the measurement of selected node voltages, which allows to define the performance of the link in terms of its efficiency, output power, and mutual inductance. The main feature of the proposed method is that it can be applied to wireless power transfer systems during normal operational conditions, without the need for disconnecting the load. The characterization procedure is validated by experimental results.