How can I run a diagnostic test on my ipad

There are no built-in diagnostic tools - but there are some third-party PC and Mac Apps that can run basic battery health checks. Popular choices for Mac are Coconut Battery and iMazing - whereas iMazing is available for PC.

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

https://imazing.com/

Otherwise, if you have any difficulties or your iPad requires service, your best option is to visit your local Apple Store or Apple Authorised Service Provider (AASP) - where your iPad can be tested and assessed as needed:

Genius Bar Reservation and Apple Support Options - Apple

Apple can run diagnostics on your iPad if you feel there is an issue. Schedule an appointment at your Apple Store Genius Bar.