typing the function name and pressing F1 helps if you have the help installed. Otherwise, first google to find which function you need (mostly fro the forum here) and then google for that function name to see in which unit it resides.
It's advisable to do that anyway if you're not sure, because things get added and move around. Like, the units that hold the same functions, but for UTF-8, or functions that don't rely on Windows, etc.
I tend to google first if I think something is difficult and could be much simpler, before I use the complex one or build it myself.