The Apple Pencil hasn't got a screen or any indicator LEDs to tell you how much battery it's got left - or if it's about to run out of power completely. But don't worry: it's easy to check an Apple Pencil's battery percentage.
If the Pencil is currently paired with an iPad, you may just have to wake up the tablet and swipe right across the screen (from either the lock screen or the first page of the Home screen) so you see the Today View and its various widgets. One of the widgets you can choose to display here is a battery indicator, showing the remaining power in the iPad itself and any paired accessories.
If you can't see the Batteries widget, swipe upwards to bring more widgets into view - it may be further down. But if it's not there at all, we'll have to add it.
How to add the Batteries widget
Tap Edit at the bottom of the Today View's list of widgets, then enter your fingerprint or passcode to unlock if required. This will bring you to the Add Widgets page.
The top list shows the widgets that are currently active, and by dragging the three-line icons to the right of each one you can adjust the order in which they appear. But we want to add a new one, so look through the More Widgets section below.
It's in alphabetical order so Batteries should be near the top. When you spot it, tap the green plus sign next to it, and it will move to the (bottom of the) top list. Drag it to your desired position in the list.
What if the Pencil isn't paired?
If your iPad and Apple Pencil are not paired (or if the Pencil is completely out of power) then it won't show up in the Batteries widget. In this case you should plug the Pencil into the iPad's Lightning port.
Firstly, it will start charging. And secondly, if and when it's got enough power to do so, it will ask for position to pair with this iPad (and unpair from any others). Confirm that you wish to paid, and the battery percentage will appear in the widget as normal, and remain there even if you then unplug the two devices.