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.

Recipe

1. Launch Terminal and type the following commands…
2. defaults write com.apple.dock showhidden -bool YES and press Enter.
3. killall Dock and 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.