How do you forecast ice?
When there is (visible) moisture in below freezing temperatures.
Unforecast ice is when you are flying in the clouds (e.g. during a hold) and the temperature is lower than forecast; or when you fly high up in known sub-zero temperatures and you encounter an unforecasted cloud layer.
You are not allowed to fly into "forecast ice", i.e. into conditions you know will create icing, in an aircraft not equipped to deal with it as per POH.