Just call the carrier the phone is tied to, they will be able to check. If they can't, avoid that carrier in the future, they cannot (or will not) protect you if you lose one of their phones.
If you find a phone, call the carrier's customer care line and tell the agent you found this phone, and ask them to check if it is active on an account or flagged as lost/stolen. For Sprint agents, it literally takes two mouse clicks and a bit of typing to check the serial number.
If it is flagged or still active on an account, it can be turned in at a local Sprint store and they can notify the owner. If it is not active or flagged, it's free and clear.