Without using Exposé, it is hard to tell which Mac OS X applications are hidden and which aren't. But, with a simple Terminal hack, you can have all your application icons displayed as transparent. Simply open Terminal.app (Applications > Utilities) and type the following command exactly:
defaults write com.apple.Dock showhidden -bool YES
Once you type the command and press enter, you need to restart the Dock by typing "killall Dock" and pressing enter. Once the Dock restarts and you hide applications, you will notice that hidden applications have a transparent look. If you want to reverse the command, just replace the "YES" with a "NO" and restart the Dock again.
Like these tips and tricks? Why not take a look at TUAW's Mac 101 and Terminal Tips category.