have never seen a forecast system for in-flight structural icing conditions, which tells you anything that is not obvious from one look at the MSLP chart.
Icing is a highly localised phenomenon, in both space and time. For 5 minutes, no ice. Next 5 mins, you get 1mm. Next 5 mins, you get 5mm. I don't see any weather model ever being any good for this.
If somebody finds a good website for this, I'd like to know.
F215 from metoffice.gov.uk offers a low level forcast (below 10,000') for the UK and F415 for Europe.
The symbol for moderate ice is a semi-circle (like a smile) with two vertical lines in the centre. Severe is three vertical lines.
See the key at the bottom of the page.