You should be able to tell from a mile away if it's fake, but assuming it's a real device I would never buy a used BlackBerry from a stranger unless they can prove that both the IMEI and PIN are clean. To do this simply have them prove to you that the PIN is good and can be registered with RIM's servers or call the carrier that the phone is from and ask them.
Also, I'd make sure that the IMEI and PIN listed in the software (found at Options > Status) match the IMEI and PIN on the stick found underneath where the battery located on the back of the device.
I'd spend five minutes looking at websites that sell the cheap Chinese knockoff mobiles like solomobi.com and check out their offerings. It is a good bet that if the Storm has dual SIMs and a TV antenna, it ain't the real storm. See also: