If you're referring to the Build System (when you press Ctrl-B), then choose the Tools menu, Build System, and the one that is currently in use will be ticked. (If you mean to open this file, then it's in your Packages folder with the extension 'sublime-build'.)
If you're referring to your ST Build then choosing the Help menu, About Sublime Text 2, will show it. Alternatively, press Ctrl-' (apostrophe) as soon as you start ST and this detail appears at the top of the Console. Andy.