If the tablet is charging see picture below – black “lightning” when charger is connected and turned on.
Notice – the red cross over the battery icon – illustrate that the tablet is not being charged but is connected to a USB cable fir data exchange – but the USB port does not provide enough power to charge).
On the web
On the web our real time status shows you if your are charging (green else a grey bar)
and here also on the web – click a table (lien) in the list shown above and find the line labelled – ChargerState:
The message when charging – see line labelled “BatteryCharging” will be “Done” (fully loaded and connected to charger) or “Charging” – or as shown False (not charging)