Checking if hardware is online

[JoergZ]

My configuration:
Raspberry Pi 2 B, Raspbian Stretch, Domoticz version 3.8153, Mosquitto version 1.3.4 MQTT broker version 3.1, Sonoff basic witch Tasmota firmware version 5.7.1

In general all works fine. But there is one thing I don't understand: allthough the Sonoff hardware is offline (not connected to AC power) , I can press the switch of the webinterface resp. android-app. How can I make sure that a switch that is offline is displayed as offline or unavailable?