As fine grained power monitoring becomes crucial in data centers, a challenge raises on how to correctly map server identities to power circuits. Power provisioning, power capping, and power tracking all depend on accurately accounting which server consumes power from which circuit. Manual survey is cumbersome and error prone. We describe a solution, called Red Pill, that can systematically and automatically identify the mapping. The idea is to generate a power consumption pattern (a.k.a. a signature) by controlling CPU utilization, and to reliably detect it from circuit-level power measurements. We describe our implementation of the Red Pill system and evaluate it with real data traces.