How to return iPad I found to its owner?

If you have found a device, turn it on and see if there's a message. The owner may have done this:

Use the Find My app to locate a missing device or item - Find your lost Apple device or AirTag with Find My - Apple Support

"What happens if you activate Mark As Lost

...You can also display a custom message with your phone number on the Lock screen, and keep track of your device's location."

Otherwise turn it into authorities responsible for dealing with lost items.

With activation lock ("locked to owner") set you won't be able to use it yourself.

You cannot. The owner has the option to provide a message on the device if it is found, but they do not have to. You will not be able to track down the owner, but the owner is able to track down their device. It is best to put it in a location where they can find it such as a lost&found or similar location where they can pick it up.