I'm not 100% sure, but I'm pretty sure. Maybe, I'll start the car, & unplug it, then see the gauge changes. But I know on most cars when the CTS goes bad, the dash gauge still reads normal. On the Audi's many people drive around with a bad CTS, & a normal reading gauge.

You, have to pull it out, & put it in hot water to see what it does. It's located next to the WP. It takes about 10hrs of labor to change it.

OEM coolant is pink. There is really no way to know if they changed it or, toped it off, why don't you call them and ask?

Yup, the car will stay hot for hours!

The thermastat, is located under that timing lock bar on the left hand side