Replace and tighten the radiator cap. Now start the car and let it run until it gets to its operating temperature (the place on the temp gauge that it normally stays at).
Turn your heater on and move the temperature control to the hottest position. Let the car run for 10 minutes with the heater on.
Turn the car off and wait for the engine to cool off. If the radiator cap or metal radiator is hot to the touch, it's still too hot to open.
*IMPORTANT SAFETY REMINDER: Do not attempt to loosen or remove the radiator cap while the engine is hot. Serious injury can result! Your cooling system is hot!