I think you can do this by monitoring the low level keyboard message while
they are sent to your app.
If I recall correctly, they have different scancodes.
The easiest way to test is to install a low level keyboard hook and watch
what is coming by when you press the different keys between which you want
to distinguish.
Mybe it is also possible by monitoring the window messages themselves with a
Message hook.

Have a look at SetWindowsHookEx
I think that is a good starting point.