You've got a few options to choose from. First, there's Spy++ utility from Microsoft that is part of their Visual Studio package. From some blog posts that I found, it looks like the Express versions of Visual Studio do not have the Spy++ utility.

If you don't want to purchase Visual Studio, you've got a few free alternatives. In the examples from Win32::GuiTest, there are spy-- and spy. Since I personally haven't tried either, I can't tell you much about either example scripts.

There's a freeware utility called WinSpy++ that is "based around the Spy++ utility that ships with Microsoft Visual Studio". Again, I haven't used it so I can't provide any more details on it. Recently, someone had posted a question here in the monastery looking for a newer version of WinSpy++ that would work with Windows 7.

Another alternative is to check out AutoIt, which is a freeware tool set that can be used to automate GUI based applications like you can with Win32::GuiTest. One of the tools that they provide is the AutoIt Window Info tool, which can help provide information about the GUI application that you're wanting to automate. In fact, I used this same tool to help find window IDs to modify the calc.pl example from Win32::GuiTest. You can look at my modified code at Re: Problems with WIN32::GUITEST.