检测用户何时截取我的程序的屏幕截图

I'm writing a Windows application. How can I tell when a screenshot is being taken of it? I read about how to detect the hotkey being pressed, but I'm more worried about the case where every 100 ms or so another program is taking screenshots of it. Namely, I'm trying to notice when someone creates another program (say a python one using ImageGrab, as mentioned in another post) which constantly takes screenshots of my app and uses them to read info off of it.I'm writing a Windows application. How can I te