I think it will vary by manufacturer. The aquarite unit I've got appears to be using a simple inductive sensor. I've not seen any schematics with the expected values - they probably don't want you fixing things when you can be replacing them.

The pool pilot units use what they call a "tri sensor" and I'm not sure at all how they work.

I think aqua-rite and pool pilot both have good reputations as far as brand selection goes.