If the turbo leak it will blow black smoke. I put RTV on my oil return pipe that connect to the oil hose from the oil pan. It clog up the pipe and black smoke was coming out. If head gasket is suspect there is a dye kit that mount on to the radiator to test for coloration change. Also do a compression test on each cylinder and compare the reading. Also if you have head gasket leak you should feel the Z losing power. Maybe it may need to run for awhile to clear up.