How do I check my magic keyboard battery percentage?

If you are talking about the magic keyboard with trackpad for iPad Pro, then it doesn't have a battery, it uses the iPad to power it via the smart connector on the back.,

The charging port on the keyboard is a pass through charger to charge your iPad Pro and leaves the iPad Pro main usb-c free for stuff like hard drives and other accessories, which can't be used on the USBC port on the keyboard