The stock temperature gauge in my S4 NA reads approximately 3/5 (just below half) on the highway. I’ve flushed the system, had the radiator professionally cleaned, and replaced the thermostat, main coolant lines, and radiator cap.

This seems a little high, but hasn't yet caused any problems. For a quick test, I’m planning to use a multimeter with a thermometer. Where is the proper location to test?