Radiator caps on vehicles rarely do fail and a radiator cap usually lasts the life of the vehicle.
But a new radiator cap is usually under $10.00 so if you're in doubt that the radiator cap is working properly you can easily and cheaply change it out for a new one.
Just remove the old radiator cap and put the new radiator cap on.
Some signs of a bad radiator cap are overflowing coolant reservoir, engine overheating, leaking radiator cap, coolants leaks due to the radiator cap not opening etc.
The radiator cap should open enough to allow extra coolant under pressure to escape the radiator and go into the coolant reservoir.
If the radiator cap spring is stuck closed it won't allow it to open up and the pressure can build up inside the radiator and allow coolant to leak out of hoses and other places.
I usually just replace the radiator cap every 5 to 10 years because they are cheap and they are much cheaper than getting a tow or damaging my vehicle.
The seal in the radiator cap may also start to degrade and crack which could cause the radiator cap to not seal properly and cause the system to not build proper pressure and cause leaks around the radiator cap.