In a King Air, you watch for ice build-up on the wings. The manual says to cycle the boots when 1/2" builds up. You also need to turn on the prop deice (electric) at the first sign of any ice. The best places to look for the initital traces of ice are on the sharper, smaller parts of the airframe like the w/s wiper arms (again, in a King Air) the OAT probe or the point on the prop spinner.