How to tell if invoked through IDLE or command line?

[Robert D. Young]

I've probably asked this before, but how can I tell within the program I'm
running if the .py files was involved by "run module" in IDLE or by using
the .py assoication with the python.exe program? I'd like to prevent running
under certain circumstances, or pop-up warnings, or change character
displays (some look different when run under IDLE).

- Robert