how to find an iphone owner with locked icloud device

You cannot do so unless the owner has, of their own accord, provided contact information with a lost mode screen message. Otherwise, there is no way to find the owner. Apple will not get involved with lost or stolen personal property so they will not even take your request for any information.

Turn it in to the police or at the least, recycle it.

Unless the previous owner has enabled this, there's nothing you can do.

Although you might think it convenient, it would be a major privacy violation if someone could somehow identify you from a lost/found phone.

If the user has reported the device as lost and configured a message to display on the locked screen, then you can use that to get the phone back to them, otherwise there is nothing you can to do trace them back.