XPLM_API int        XPLMWindowIsInVR(
                         XPLMWindowID         inWindowID);    

True if this window has been moved to the virtual reality (VR) headset, which in turn is true if and only if you have set the window’s positioning mode to xplm_WindowVR.

Only applies to modern windows. (Windows created using the deprecated XPLMCreateWindow(), or windows compiled against a pre-XPLM301 version of the SDK cannot be moved to VR.)