If you look at the center frame of an A320's cockpit windscreen you'll see a small metal rod sticking straight out of it. It looks like a 3 inch long nail.
I asked about this object in this forum a few years ago and was told it's a probe for ice detection.
I can't remember if the pilots simply look at this probe to visually determine if airframe ice is forming or if it's an electronic type of sensor for the EICAS like Philsquares mentioned. Maybe it's electronic and causes only the windscreen heat to automatically turn on if it detects ice.
You can see the A320's ice detector on the windscreen in these 2 photos.
>> Philsquares, where's the ice detector(s) located on the 744's that you fly?
This photo shows there's no ice detectors on the 744's cockpit windscreen like on the A320.
View Large View Medium
Photo © Helmut Bierbaum