How to tell if a iphone 5 is stolen or not?

One day my brother just came home with a brand new iPhone 5 (Box, Ear phones, everything was brand new) can't really recall how he got it. He had money to buy it, but I'm not sure if the previous owner stole it and sold it to my brother. My brother hangs around a lot of negative people and he's not exactly streets smart. Ever since he got it, he's been trying to tell me to sell it on eBay for him. My guts keeps telling me it's stolen.

It's a sprint phone.
How can I tell if it's stolen.

If it's brand new in box unused than i don't think there is a way to tell.

It is obvious that you believe it is stolen. If someone tries to activate it & the ESN is bad… Then it is stolen.