How can I measure moisture on a conductive flat surface as an analog/digital value?
I agree that measuring dew point is likely easier (buy a humidity sensor and do a bit of math), but if you want to measure condensation, consider using capacitive sensing.

Water has a dielectric constant much higher than that of air, so the capacitance between two plates will be higher if there is some water between them rather than merely air. (If the water is impure and therefore conductive, it instead reduces the effective distance between plates, also increasing the capacitance.) This is the technique commonly used soil moisture sensors.

Measuring capacitance can be done in many ways: I might use a hysteresis oscillator.

Note that you will need two conductive plates for this technique, not merely one, but they can be coplanar rather than parallel if that is easier to arrange.