How to see if found AirTag has a message

You can do the following:

If you have found an unknown AirTag, you can follow these steps to see information about it, or to disable it:  

1. If you have found an AirTag, hold the top of your iPhone or NFC-capable smartphone near the white side of the AirTag until a notification appears.
2. Tap the notification. This opens a website that provides information about the AirTag, including its serial number and the last four digits of the phone number of the person who registered it. This can help you identify the owner, if you know them. You might want to take a screenshot to make sure that you can document the information.
3. If the owner marked the AirTag as lost, you might see a message with information about how to contact the owner.

What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support (MY)