XPLM_API int        XPLMWindowIsPoppedOut(
                         XPLMWindowID         inWindowID);    

True if this window has been popped out (making it a first-class window in the operating system), which in turn is true if and only if you have set the window’s positioning mode to xplm_WindowPopOut.

Only applies to modern windows. (Windows created using the deprecated XPLMCreateWindow(), or windows compiled against a pre-XPLM300 version of the SDK cannot be popped out.)