While I rarely drink hot coffee, there seems to be a sweet spot between scalding and lukewarm, where it doesn’t burn but emits a sufficient amount of heat when imbibed. You could take a guess at this temperature by the steam coming off of the surface of the liquid, and how long it’s been sitting. For a more direct way to sense this vital stat, YouTuber Make Fun Stuff has come up with a non-contact beverage temperature monitor.
The device holds an IR sensor on top of the cup, using a crane-like linkage structure that's soldered out of 2mm brass rods. In addition to providing mechanical support, these rods transfer electrical signals to the sensor, as what turned out to be a beautiful angular circuit sculpture. Bent wire was also tried here in multiple configurations, but this acted more like a spring than the rigid platform that was needed.
Temperature info is piped to an Arduino Nano board inside the base, which is powered by a 9V battery, cleverly friction-fit in place with a block of wood. It’s turned on by the weight of the coffee cup via a small switch embedded inside the base, and it uses five LEDs that don’t quite poke through the wooden base to display temperature. This means that lights disappear when not active, which makes for an understated look that goes along nicely with the sculpture-like sensor overhang.