There are several ways to test a head-gasket for failures.
A chemical test will turn a CO2 sensitive fluid a different color if CO2 is found in your cooling system.
You can pull your spark plugs and pressurize your coolant system for 24 hours. If you turn the motor over and there is coolant shooting out of the spark plug tubes, you have a slow leak.
You can inspect plugs for signs of coolant.
You can monitor cooling system pressure and thermostat opperation via a pressure guage and scan tool....
Point is, have someone that really knows their stuff test the head gaskets a few different ways before going down this road.
Also check airflow over the radiator. Put a fan in front of the radiator and see if temps come down at idle. If this solves the issue you have a weak fan or a clogged/weak radiator.
If you rev the engine to 2000 rpms while sitting still and the temps come down, you have a flow issues. Weak water pump, restriction in the system, etc.
If the system starts to overheat after a few minutes and nothing seems to bring the temperatures back down you may have a headgasket issue or a stuck thermostat.
The thermostat should be replaced if it's been subjected to high temperatures. They are simple springs and wax valves and spring metal can loose its memory with high temperatures. A new thermostat may be opening incorrectly but you'll be able to see that with a scan tool.
2001 Disco II SE7
ASE Master Technician