How to detect distilled water?
I like Russell's suggestion of optical methods.

Another approach is a capacitance meter : water has relative permittivity of about 80, therefore measuring the capacitance between two insulated plates a fixed distance apart will show if there is some high permittivity liquid between them. It's fairly independent of water purity.

Couple of drawbacks : 


permittivity is temperature dependent (88 at 0C, 80 at 20C, 55 at 100C) so precision (e.g. using it to estimate depth, or the proportion of the plates submerged in water) would need some knowledge of temperature to compensate the error. 
Cannot easily distinguish water from other high permittivity liquids, although alcohols and hydrocarbons appear to have about half the permittivity of water
A bit more complex than a simple resistive measurement.