A bad radiator cap can sometimes be detected when coolant is leaking around the edges of the cap. Sometimes, it's possible to hear antifreeze bubbling when a radiator cap is bad. If this occurs, wait for the car to cook down before opening the hood. A bad radiator cap can also be tested with a pressure gauge.
When the pressure numbers on a pressure gauge start to drop while testing a radiator cap, it means the cap isn't working correctly. Unscrew the radiator cap, clean off any debris then retest it. If the radiator cap still shows a pressure drop, it needs to be replaced.
Another common radiator cap problem involves the spring-loaded plunger of the cap being jammed. When this occurs, coolant can't move between the cooling system and the overflow reservoir as it should. A visual inspection of the overflow reservoir should show whether this is happening. Another symptom of this problem is leakage of coolant around the radiator cap. If the cooling system has just been filled, however, the leakage may actually just be some spillage resulting from the refilling process.
Radiator caps are easy to replace. Just buy a new cap with the same pressure rating, and screw it in tightly. Make sure the car is completely cooled down before replacing a radiator cap.