How can you tell whether the rear fog lamp is switched on?

You answered correctly. In German vehicles, when the rear fog lamp is switched on, it is indicated by a red indicator light, not amber or blue. Amber indicator lights typically signal the use of indicators/turn signals or hazard lights, while blue is used for the high-beam headlights. Always look for a red indicator on your dashboard to know if your rear fog lamp is on, as this helps you avoid dazzling other drivers in clear conditions.