Detection of condensation on automotive glass
The best and most dependable way would be an infra-red detector. Put the IR LED outside the window in a bottom corner, and the sensor on the inside of the car window positioned so the IR LED hits it. Leave a 1 inch gap from the IR LED to the glass so any condensation will build up there.

Use infra-red (or red) because water molecules block/absorb the red colors the most. Water is somewhat transparent to green/bluish light so they would not work unless you wanted both for an 'A vs. B' comparison. Or use an IR LED on both bottom corners of the window for an 'A vs. B' comparison.

The sensor should have a voltage output that drives a simple display on your dashboard, and be powered by 12 VDC if possible, just to keep things simple.

Calibration consist of measuring the level on the display with a very clean window and IR LED lens or cover. To get consistent readings be sure the window and the IR LED cover stay clean. Fog/condensation will cause the reading to be lower. Condensation also attracts dirt and dust so you may have to clean often if fog and condensation happen often.

EDIT: You will need to pay attention as to how the weather other than fog affects your readings. If you live up north the UV index can be high (maybe a small sun shield?) and the freezing cold may cause readings to drift. In the south the intense heat may cause a drift in the readings, so you will have to learn how it behaves with just fog and condensation vs. how other types of weather affect your readings.