I found a lost watch

Are you able to turn the iPhone on? If the owner provided information on the lock screen about the device being lost or to call them, you can. Otherwise, there is no way to find out who the owner is. You can turn it in to lost and found if you found it in a public place, or turn it in to law enforcement in the town where you found it.

Unless you see a screen like this;

There is nothing you can do,hand it in to the police.