Water throwing out of relief valve on boiler. Believe problem is related to the built in expansion tank. How can I prove this before I get an expert in?

MCM, October 2010

If the pressure rises significantly when the heating is on, do the following check:
Turn off the electricity for safety's sake.
Top of boiler at the back, the expansion vessel is large and red, on it, you will find a schrader valve (very much like a car tyre valve), if you press it in briefly and no air comes out, the expansion vessel needs charging.

Carefree, October 2010