A simple thing would be to take off a valve cover and use a dial indicator on one of the valve spring retainers to measure lift. If it's more than the stock lift, you have a replacement cam, unless it has higher ratio rockers also. Also, check an intake and an exhaust valve. Most aftermarket cams are a dual pattern with more exhaust lift than the intake, if the lifts don't match it's aftermarket.
you have anybody else with a gt around you? Maybe compare the exhaust... If your really wanted to know you could take the time and see what the specs are on the cam... (fairly long drawn out process)