How do you check iPAD battery health?

Currently, you don't directly from the iPad.

iPadOS 14, currently, has NO battery health indicator, like on an iPhone.

It is absent from iPads and iPadOS as iPads have much larger and longer life internal batteries than any iPhone.

Sorry.

There’s no direct means. Time the battery run-time and usage. Or install something akin to CoconutBattery on your Mac, if you have a Mac handy.