Many boilers have isolation service valves installed on the underside where the heating pipework connects to the boiler fittings. These may only be seen as screwdriver slots in the elbows but allow the boiler to be isolated from the heating circuit.
With the boiler off, turn them off and wait for the pressure to drop. This will take a fair while. Also, by opening a bleed valve in an upstairs rad, you can see how long it takes the pressure to be lost in the heating circuit. Because there isn't an expansion vessel on the circuit any leak will result in lost pressure very quickly. If it is a bad leak 20 - 30 mins is enough to lose all pressure so nothing will squirt out of a bleed valve.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!