Stolen/lost iPhone 4 i their way it's technically stolen?

How can i prove a phone is mine if they cleared all the data and the only thing i know is that the back of the screen is broken… I lost the phone yesterday and then on Craigslist they put a add on it today and i believe its my phone because they said cracked screen… This phone was giving to me by someone so any other info i can't get… Now i believe someone stole the phone from em when i was in the store talking a sales person i must have placed it down and got distracted and i think they snatched it:/ the only other place i could have lost it was on the bus… What should i do?

Added (1). I do not have the box or anything. I was hoping their are other ways to get info about the online or other ways unfortunately i didn't take the time to add locks and other things. I was hoping their was something in the phone that you can put that will never change even if reset…

You could look on the box of the phone, and find the UDID, IP address, and other info and compare it to the phone to see if it matches.