How can I verify that the protective earth/ground is separate from the neutral wire in an electrical AC installation?
Usually your central heating and your water piping is grounded by regulation. 
Your ground wire shouldn't have any potential differerce with these installations (as there shouldn't be any current flowing through it), whereas neutral may carry a few volts (because of current elsewhere in the system lifting potential). 

If your installation is behind a residual-current device and you have the live wire identified, another way to figure out neutral and earth is connecting a 25W lightbulb. If the bulb lights up you're using neutral, if the residual-current device trips, then you used earth.

Of course if you don't trust wiring or its colors, the only way to go is to pull the existing wires out of the pipes an do a proper re-wiring according to national regulations.