The outsourced manufacturing of integrated circuits has increased the risk of intellectual property theft. In response, logic locking techniques have been developed for protecting designs by adding programmable elements to the circuit. These techniques differ significantly in both overhead and resistance to various attacks, leaving designers unable to discern their efficacy. To overcome this critical impediment for the adoption of logic locking, we propose two metrics, key corruption and minimum corruption, that capture the goals of locking under different attack scenarios. We develop a flow for approximating these metrics on generic locked circuits and evaluate several locking techniques.