You have to remove the water pressure and water to check the air charge but a quick would be to see if there's water at the tire valve on the expansion tank. If you check the air and get water then the expansion tank bladder has failed.

There are still many ceiling hung tanks used today. If you have one, it may need to be drained. It can be messy, so let us know.

You can also have a pinhole leak in a domestic water coil or a faulty pressure reducing valve, or not set properly.

If the pressure builds when the boiler heats up or when the boiler supply valve is closed, it's probably the expansion tank.

If the pressure builds up with the boiler supply valve closed and the boiler is cold, then it's probably the coil.

If you close off the inlet and outlet to the coil, and the boiler is cold, and it still builds pressure, it's probably the pressure reducing valve.