Take the phone to a Verizon Store and have them run a check on it. If it turns out not stolen or reported lost, they will do all the work for you for no charge.
Or, if you'd like, you can call Verizon and do the same thing. They will ask you a few questions like some of the numbers off the back of the phone, and run the checks on it, once again, if all's good, they will activate the phone to your number.
If it is stolen or reported lost, you are out of both a phone, and the money you spent on it.....and maybe something worse if they call the authorities.