This palm-sized cherry-red water sensor is simple and easy to implement in your wet-dry-sensing project.
Usage is very simple: connect the minus (-) pin to ground, connect the plus (+) pin to 3 to 5V DC and then the output signal pin (S) goes to your microcontroller. The sensor has a very lightly biased transistor that is connected to two long sets of interlocking traces. When a little bit of water/condensation lands on the PCB and touches two of the traces, the transistor will turn on, setting the output pin high. The output voltage does vary a bit with how much water is detected, but it is recommended just using it as an "on or off" type sensor.
More information is available on the website of Adafruit.