How do real pilots determine where the cloud layers are to avoid them on their desired flight level? I know that GA on some occasions can request a block altitude but what about airliners? As far as I know it’s hard to actually measure where the top of the clouds is but I can’t imagine anyone going to cruise thru them with decing on all the time.
Thankfully high-altitude flight planning is a lot easier. Can get a basic idea beforehand by looking at other reports (PIREPS) of turbulence, or if dealing with thunderstorms, tops are depicted on the radar summary. Most jets cruise pretty close to the tropopause, so that helps with the air being pretty stable.
Normally at cruise altitudes, it will actually be too cold for ice to form anyway. The flight manual will likely only require anti-ice systems to be on if the temperature is warmer than -40. So not usually a factor at cruise. But if you do get stuck down low in icing conditions, not a big deal to keep the anti-ice on for a while. Usually only hurts performance in the climb. Severe freezing rain / SLD is an exception to this.
I can’t speak for the rest of the world, but in the United States, Federal Aviation Regulations require you to know “all information” about that flight, including weather. You do this before the flight either through a weather briefer, online aviation weather resources like aviationweather.gov, or an app like ForeFlight. While flying, you can call a flight service station that will give you updated weather enroute. Airlines will have their own systems for disseminating this information. And all pilots, whether GA or airline, have the ability to issue pilot reports, or PIREPs, that give current conditions for a particular area.
Throughout PPL training and after, trying to get a good answer for this specific question remain elusive.
A good weather briefing, and fresh PIREP’s are a good start.