No scientific answer, but I've always kept an occasional eye on the temp gauge. I'm sure in the past it was always rock steady in the middle (90C I think) after warming up. However, this winter I have noticed that it often drops below the middle, not a lot, maybe 10C maximum drop, and then goes back up. I think it happens when the car is coasting more (eg going downhill on the motorway) where the engine would produce less heat. However, I would expect a good thermostat to compensate for that by regulating the water flow through the radiator.
It clearly works in general as it doesn't deviate much, but I'm sure it didn't deviate at all in past years.