How do you assess control loop stability?

Stability assessment methods: time-domain analysis (observe oscillation, damping ratio - quarter decay ratio target, overshoot percentage), frequency-domain analysis (Bode plot showing gain and phase margins - typically >6 dB gain margin, >45 degree phase margin), and empirical indicators (decreasing oscillation amplitude, settling within acceptable time). Unstable loops show increasing oscillation amplitude or runaway response. Loop stability depends on total loop gain - product of controller, process, valve, and sensor gains. Reduce gain or add filtering if marginally stable.