Adorama, like Amazon and B&H has a pretty much no questions asked return policy for the first 30 days. I have only used it once, to send back a used lens I could never quite get to tune in on my camera. They took it back with no questions asked.
As for testing the camera, I would think it was checked out pretty well by Nikon. I would just take it out and shoot like I normally would. Try the setting you normally use. If you have access to a grip, try it. Try flashes on it.