How can I verify an iPad has been wiped?

Your iPad has an encrypted file-system that can only be accessed using the correct encryption keys - the only copy of these unique encryption keys being stored within the iPad's security chip (Secure Enclave). Entering the correct device Passcode unlocks the the Secure Enclave and releases the encryption keys to the Operating System.

"Wiping" your iPad instructs the Secure Enclave to destroy the encryption keys - this being known as a crypto-erase. Once erased, all locally stored data is permanently beyond reach and cannot be recovered.

Following this process ensures that your iPad is properly erased prior to disposal:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you mean all data and all account information wiped, then when you power it on, you should see the multi language Hello screen that invites you to set it up.