How to watch for a print screen system call?

A process is taking pictures of the screen. (screenshots, print screen)

How can I watch for a windows print screen api call so I know when a screenshot was taken?

I don't care from what process the screenshot was taken.

Do I have to necessary attach a debugger to the process taking screenshots?
Assume I can't attach a debugger.

I would like to do it in C#.
Something like....

while(true){
    if(lastWinAPICall == PrintScreenCall){
       // screenshot taken
    }
    Sleep(100);
 }
 

Sorry, I don't know a lot about low-level stuff.

Thanks

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