On vertical tank radiators, it's easy to buy a deep fry thermometer and attach a tether. Remove the radiator cap, start the car, warm it to operating temperature. You can check the temp in the tank and will see when the thermostat opens. This won't give you engine temperature but will give you a good idea of your hot idle coolant temperature to simulate cruising and stuck in traffic conditions. If the coolant in the radiator is over 200 degrees you're not cooling well. I have an infrared thermometer that can be calibrated when the engine is cold, then will accurately indicate the block and head temps when the engine is warm. Mine was over $700 when bought 15 years ago, but they are now down to $75 or so. Mine is capable of measuring exhaust manifold temps, though, and the cheaper ones only go up to 500 degrees. I can also use it to detect a misfiring or intermittently firing plug, because that plug will be cooler when pointed at.