A radiator shop has a tool to pressurize the water system. you can also buy the tool from Snap-On. pull the plugs and pressurize the water system. if it comes out the plugs you have a bad head gasket.
If you can see bubbles in the radiator buy running with the radiator cap off it also has a bad head gasket. NEVER take the cap of the radiator if it has been running. start with a cool motor, take of the cap, run motor to see if there are any bubbles are white foam in the radiator which indicate oil in the water.