If you find an iPhone or iPad, you can ask Siri, "who owns this iPhone", and she will pull up the contact information of the owner. Some people may view this as a violation of privacy, but if my iPhone is lost, I'll be grateful if someone tries to locate me using Siri.
Siri is only able to identify the owner if they are identified as the owner. Here is how to identify yourself as the owner of your iPhone or iPad:
Go to Settings > Siri > My Info > and select your contact record.