Finding the answere is harder than I though... but interesting.

I searched a few web pages to get any baloon example (not found any), and then started trying normal applications, like Windows Character Map. Maybe this will help you in any way.

My main though is that balloon is a window like any other window - find its handle and you'll get the content.

This description + Wind32::GuiTest::Examples was the base. You can find there spy.pl complete and working example. I only added sleep 2 before loop and after running script (output redirected to file), I moved mouse over Total commander icon. On the result list, there appeared line

+          66316     , 'TTOTAL_CMD               ', 0         , Rect:58 ,73 ,1199,938   'Total Commander 6.54 - RTS Networks Sp. z o.o.'

where the last string was my balloon text. Similar test with quick launch icons didn't succeed. TaskBar icon's balloon worked like, too, and this is example output from some button's tooltip in application itself:

Depth      WindowID  , 'ClassName                ', ParentID  , Rect:W
+indowRect,   ,   ,      'WindowText'
           0         , '                         ', n/a       , Rect:n
+/a,   ,   ,      ''
------------------
+          4326422   , 'TToolTip                 ', 0         , Rect:3
+15,150,355,167   'Pack...'
[download]

where 'Pack...' was the balloon text.

I didn't manage to achieve the same results with win 'Character Map' application, it seems that success depends strongly on how the application was written.

GuiTest modules have possibility to move mouse programmatically, so you could try to combine this all together. I hope there is easier way and anyone has better advice for you.