If it is not the original iphone (the bigger one), it has a feature called "find my iphone". You activate it from any computer and the gps tells you exactly where the phone is located. It has to be turned on of course. If not, it will give you the last coordinates. It is a great feature, and I used it once to find my iphone that fell out of my pocket into the bushes at the local park. Now before you ask, I was retrieving a baseball my son had thrown over my head.
Anyway, the suggestion to just call one of the numbers on the phone is best. Also, in the contacts of the phone there will be a contact listed as "me". This is the guy who owns it!
Bring it back and get your own!