MainWindow focus

Pardon me for the lack of better terminology.  I need to determine from 
within a Tk application if it is currently the top level task/process in 
Windows.  I suppose I mean that MainWindow is currently focused instead 
of any other windows application.  I am not finding a method that I 
could use such as in Tk:Widget.  Is this something that needs/can be 
acheived with an API call?
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