Blade Icing
If you encounter ice and are over, or near the ocean, get over it and go down to about 200-500`and your ice will in most cases melt of the blades.
If you fly innland and encounter ice, get down on the ground and wait out the weather. Freezing rain should be avoided at all times!!
To your main Q. The best way of recognising ice is the Tq. If you are temporaly in a cloud and expect to brake out in a few minuts, then leave the collective in it`s position as long as the Tq. is withing the green arc. This is done to have controll over the ice melting of the blades. If you lower collective, you will not have the same controll over how much ice you still have on the blades.
You will have to take in to consideration a lot of factors to do this, like aircraft weight, altitude, mountains, area clear of ice(FC). This is anyway how I was tought to do it from one of the pioneers in offshore flying.
Hope I helped a little......