There are different operators that handle the anti-ice thing differently. But it also depends on the aircraft. Dew point spread is a very common way of defining visible moisture, but always comes as a combination with something, i.e. OAT.
Some airlines define it with a visibility (say 1.5km) plus a temperature. If your spread is small, chances are that you have bad visibility. This of course can only be obtained close to an airport (where dew points are regularly transmitted).