I don't think there is a practical solution to this. There is no standard API for this stuff.
If this was at all possible, this is how I would do it (there may be a better way).
Take the basic code from a window finder, such as
FindWindow[
^]. Mix that with something like
Microsoft's SAPI[
^].
This will at least get you the control which is under the mouse. Get the window text and shove it into the SAPI.
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