Take a look here:
http://www.dadams.dsc.net/flying/wca/photos/index.html
Scroll down and on the left hand side there is a picture of ice forming - click to enlarge it. As you'll see, on such a small protrusion (where, as others have said, the ice forms first) the ice is clearly visible because it changes the shape of the structure. Tougher in the dark or if your windows are frozen over, but if your windows have frozen over you can get bet you'll have ice elsewhere.