How do I find the owner of a lost iPad?

Whilst your desire to return the iPad to its owner is commendable, Apple will not assist with locating, or reuniting a lost/found/stolen device. Using any information to which Apple may have access would likely violate privacy laws/legislation that apply in many countries.

If you can turn-on the iPad, if registered as “lost” by the owner, the iPad mightdisplay a message or owner contact information. If so, you may be able to independently make contact - otherwise, you’ll not have any means by which to contact the owner.

As such you would be best advised to simply hand-over the iPad to your local authorities - as they are best placed to reunite the device with its owner if a loss report has been made.

you pretty much can't, and if you could then it would not be apple helping you if they kept customer data tied to purchases and provided them to people they would be in cross violation of laws in most of the world.

you can try reading

Found an iPad, now what? - Apple Community

Keep the iPad charged and maybe the owner can track it. If not you have a new pap weight.