I think we have an impressive example of a false problem.

If you want to clear console window in portable manner, use portable direct console i/o library (pdcurses, for example, or others). That's the only right answer. May be a suitable ersatz of such libraries is <conio.h> stuff implemented with minimal variations in most of C and C++ programming systems. If you need to use standard console i/o (stdio.h or iostream), never use clear screen operation in your program interface design: no such entity as a screen in C and C++ stream i/o models.

As usually you can't run executable module on different OS in native mode, so the question "is there some way to find which OS is being used at the moment of execution so the program can decide..." is a senseless sentence. The obvious answer is "your program compiled and linked for OS1 can't run on OS2 so it find nothing on OS2, moreover it can't find itself on OS2".

It's the other question: "How to write portable sources which can select proper pieces of codes for different OS by C++ conditional compilation features?". Download good portable library (or system) and look at its configuration header. Of course, it looks like Ancient Dragon's example. But it's the other story and it's not so simple in common case.