keyboard charging

In the System/Bluetooth settings pane the amount of charge for the keyboard will change to the same icon but with a lightening bolt in it when plugged in as shown in this animated screenshot:

I can't recall what information was presented in MacOS 10.13 but in my Sonoma system if you have the Bluetooth icon in the Menu Bar you can click on it to show the battery level of the Bluetooth device. If it is charging you will also see the same sort of charging designation as on an iPhone which is an energy or 'lightning' symbol.