The radiator has to drop the water temp to less than the thermostat.
You can read the top, middle and bottom temp of the radiator with a IR thermometer to see if the radiator is cooling properly while the engine is idling and the fan blowing air through. If the radiator get hotter than the thermostat and has good air flow you have a radiator cooling issue. A leaking head gasket will also cause such on some rigs. On some if you remove the thermostat and it runs cooler suspect a leaking head gasket.
You can read the top, middle and bottom temp of the radiator with a IR thermometer to see if the radiator is cooling properly while the engine is idling and the fan blowing air through. If the radiator get hotter than the thermostat and has good air flow you have a radiator cooling issue. A leaking head gasket will also cause such on some rigs. On some if you remove the thermostat and it runs cooler suspect a leaking head gasket.