Martin:

Are you familiar with cooling system pressure testers such as this:

Stant St 255 Cooling System Pressure Tester in Case | eBay

There are various versions of such a device for pressurizing the system, but all allow you to pump up to the desired pressure and observe any change on the gauge. An intact system will hold the set pressure for many hours, affected only by changes in temperature.

The gauge on the tester will also display any increase in pressure when the engine is running; a rapid increase may indicate a head gasket leak into the cooling system.