how to run diagnostics on an iPad?

There are no diagnostic tools on your iPad - other than a client that provides no functionality at all until connected to Apple’s diagnostic systems.

If your iPad requires diagnostic tests, you’ll need to you seek assistance directly from from Apple Support. You can contact the Support Team using the Get Support link at top-right of this page.

You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad as necessary:

Genius Bar Reservation and Apple Support Options - Apple