When you launch an app, it shows up in your Dock with a small dot underneath it, but there is no way (by default) to tell which app is hidden. You can, however, enable a hidden setting which makes the hidden app’s icon transparent, like Tweetbot and Ulysses in the screenshot above.
- Launch Terminal and type the following commands…
defaults write com.apple.dock showhidden -bool YESand press Enter.
killall Dockand press Enter.
If you’d like to remove the icon transparency when apps are hidden, then just use Terminal to execute the following command:
defaults write com.apple.dock showhidden -bool NO.
This works wherever you place your Dock, though we all know its proper place is on the right side of the screen.