The invention indicates a correct one of multiple rotor positions. For example, the invention can determine, in a PM machine, which of two possible positions a rotor is in when the position is known only within .+-..pi. radians. If the actual rotor position is known to be one of .omega. and .omega.+.pi., for example as indicated by a resolver, the proposed invention can be used to determine whether the actual position is .omega. or .omega.+.pi..