Watch the fan exhaust. A healthy winter plume is wispy and dissipates quickly. A dangerous plume is thick, heavy, and drifts horizontally without rising. This indicates the water is entering the cold air basin at a temperature too low to melt the ice forming upstream.
Here it is, the line you should memorize and stencil onto the tower control panel: cooling tower handbook
Respect the cold. Your tower will thank you in July. Watch the fan exhaust