on real life pilots are always sacanning visually for icing formation on the wings when conditions exists.
on the sim there is no visual clue, but ANTICIPATION is key. Anytime you are flying <10C TAT and humidity exist (clouds, rain, snow, etc.) turn on anti-ice systems until icing conditions no longer exist.
Icing is extremely dangerous on real life as well because aircraft looses aerodynamics and gains weight, you are basically a piano falling from the sky. Anticipation is always key.