I would like to determine whether there is already a compiler installed.
Do you know a command on Windows which I could use in the console to do that?
There is no catch-all way to discover whether there is "a" C or C++ compiler installed, because different compilers are invoked in different ways. You could iterate through the common ones (e.g. running g++ from shell) but, really, you should know what software is running on your computer because you installed it. If you did not set up the computer, ask the person who did.