What has to be the temperature difference between a surface and air for condensation?
Good question. Now remember first that the dewpoint is the temperature to which the air must be cooled to produce saturation, or 100 % humidity. Cooling any object upon which condensation is desired would mean bringing it down to the dewpoint temperature such that conduction from the object to the adjacent surrounding air would bring that air to its dewpoint and hence produce condensation. To determine this dewpoint temperature you need an instrument which will produce the dewpoint. This instrument is the hygrometer. For example the dew-point hygrometer does exactly what you are seeking to do. It measures the dewpoint temperature by cooling the surface of a mirror until condensation forms.
