When you activate this feature any application that you have hidden will become transparent in the Dock, making it clear which applications you have hidden, and which ones you still have open.
Here’s what you need to do.
Open the Terminal
To enable this feature copy and paste the following into the Terminal window:
defaults write com.apple.Dock showhidden -bool YES
Then reload the Dock by typing in the following:
The Dock should reload and the effect should be active.
If you want to turn this feature off later, you can do so by opening the Terminal and typing in:
defaults write com.apple.Dock showhidden -bool NO
Then relaunch the Dock again.
Tip thanks to OS X Daily.