How to check my charge status on my Apple Pencil gen 1 when it’s charging by adapter

To see how much charge your Apple Pencil has left while you're using it, check the Today View on your iPad. Just swipe from left to right on the Home screen or Lock screen.

source: Charge your Apple Pencil and check the battery - Apple Support

In addition to monitoring the charge status of your Apple Pencil in the Today View, using iPadOS Batteries Widget, you can also see the charging/battery status of your Pencil directly from iPad settings:

Settings > Apple Pencil