Is there a way in modern vehicles to know when the thermostat is on vs off? Is it possible to tell when you don't have access to under the hood, IOW when you're driving?
You will know the thermostat is open when the engine reaches operating temperature. The thermostat opens just before or around the operating temp of the engine. Most modern vehicles have some sort of temperature gauge in them. If it's just a needle, you'll notice where the needle rests under normal operating conditions (most that I've seen sit right in the middle of the gauge, give or take a few degrees of arc).
If you don't have a temperature gauge, you can tell when the thermostat is open for sure by noticing the heat coming off of the upper radiator hose. It will become very warm very fast, so watch when you touch it your hand doesn't linger or you will get burned. Your radiator will start heating up at this point as well.
If you radiator has a cap installed directly on it, you can start to see radiator fluid being circulated in the end tank of the radiator associated with the cap.
One last thing, if your vehicle has a separate pressure tank with the radiator cap installed on it, you should be able to start seeing the coolant circulating through it when the thermostat becomes open ... that is as long as the tank is clear or semi-clear.
Also of note is that after the engine is up to temperature, the thermostat will be in various states of "open", meaning, it will always be open to a state.
my car seems to heatup in city trafic. I thought gauge was bad cause it show red n green light only 4 hot n cold. So bought obd2 and found gauge is correct. 2nd guess was bad thermostat but seems At 190 F does open n bring down to 185 but on next stop in trafic temp reaches 192... Thermostat again tries and bring back to 187 F. And after few stops n temp going down n up the temp reach 225 F and red light get on. No idea how to remedy this. Coming to question i found there is no physical sensor attached to thermostat. And you cant know open or close in real time. Only thing is to guess by watching temp through obd. But it is not exact because temp sensors are outside of engine. Other way is to open the housing pull out thermostat n test it on stove.