Get one of the infra-red temp gauges that registers when you point it at something and pull the trigger. We used those things to check tire temps on the race cars after a hot lap, and they're very accurate. Just point & click at any surface, and you get an instant reading. The coolant outlet neck above the termostat, the radiator tank, or even the coolant in the top tank with the cap removed would be a pretty good source of reference. The tool isn't that expensive, unless you need some outrageously high temp capability.
Jerry
