First, non-tech solution: Check the manual. Any decent device should have this stated there somewhere. That will also help you to confirm the required voltage, since in my experience barrel jacks for 5 volts are rare. Also name the device, maybe someone here knows the polarity.
Otherwise, open it up and check connectivity from the jack to any exposed shields. Those are generally at GND, so would be connected to the negative input.
EDIT: See the latest EEVBlog (IIRC #1071) for example. At the beginning, Dave shows how he determined the polarity of the barrel connector.