In this paper, we present an approach using relay feedback for automatic identification of the PID configuration and parameters in existing and operational closed-loop systems. During the duration of the same experiment, the control performance may be evaluated, providing an assessment of whether the current control parameter set is adequate and if re-tuning is necessary. Systematic and on-line automatic PID tuning rules are further developed and provided for common configurations of PID control. Simulation examples and a real-time experiment are provided to illustrate the practical appeal of the proposed method.