You can use a program called PhotoRec and/or testdisc to retrieve all the previous data that had ever been stored on the USB stick to ensure it is yours, if your planning to just return it i can't help.
I have used this program a few times before, last time i used it on a 4gb flash drive, i retrieved 12gb worth of data it was a random one i had found in my house but it was empty, after getting all that data i found out who's it was, i returned it to them and they where a bit shocked that all that information was still on there from years and years ago.
If you want to protect your identity format often!
I tested this on a friends 2gb flash drive, found out their bank details, their work roster, their old address and new one, they where horrified. Very powerful tool i love it.