If there is no expansion tank, get one. If there is, was the air pressure in the tank checked before installation? All water pressure must be off the tank before checking. If for some reason the tank had 30 PSI in it, it's not a tank, its a plug.
Shut off the water going to the boiler. Open the boiler drain and drain a few cups. If the pressure doesn't change to speak of you can assume the tank is OK. Now, make the pressure 15 pounds shut off the water & check it in a few days.