If you need to detect rain drops, or water in general, sensors are available that are quite capable of the task. However, according to Tindie seller cowtheory, these sensors are only good at detecting large droplets of water, not so much the much the finer layer of moisture droplets formed by condensation. After unsuccessfully searching for such a moisture sensor on the Internet, cowtheory instead came up with the Water Condensation Sensor board, a unique device configured specifically for this task.
The device is literally a small circuit board with very thin (.2mm) traces, allowing it to sense tiny condensation droplets. Using it with a microcontroller with it is easy enough; connect one pin to ground and the other to an analog or digital pin set as a pullup input. When condensation hits the sensor panel’s surface, it connects the traces just enough to pull the pin to ground, allowing the microcontroller to detect this state and take action as needed.
One could see such a sensor used to detect condensation from a shower, or perhaps to see when your metal mailbox is dry enough in the morning to ship out (Tindie) packages without residual moisture. Of course there are a huge number of other projects one could do with such a sensor. Perhaps it will spark a new idea, or even revive an old one!