How can I check my iPad's battery health?

Unlike iPhone, iOS/iPadOS for iPad has never had the Battery Health feature - most likely because iPad batteries are both considerably larger and have a much longer service life. 

An iPad battery is rated to maintain at least 80% of its initial capacity after 1000 full charge/discharge cycles - whereas an iPhone is rated to have 80% of initial capacity after 500 full cycles.

There are software packages available that can access battery cycle count. Popular choices for Mac are Coconut Battery and iMazing - whereas iMazing is available for Windows PC.

https://www.coconut-flavour.com/coconutbattery/

If you are experiencing difficulties with your iPad battery holding charge, you might be best advised to first have your iPad tested. 

If you directly contact Apple Support - and explain your problem - you can request a remote battery diagnostic. The tests will confirm the status of your battery - and other elements of your iPad’s health.

Alternatively, a visit to the Genius Bar at your local Apple Store would provide opportunity for your iPad to be similarly tested by the Apple technicians - after which you can explore your available options.

Genius Bar Reservation and Apple Support Options - Apple

You can contact Apple Support using the Contact Support link at top-right or bottom-left of this page. You can also initiate a support case using the excellent Apple Support App; if not already installed on your iPad, the App can be downloaded from the App Store:

https://apps.apple.com/gb/app/apple-support/id1130498044