Enabling Scroll Bars In Mountain Lion?
Terminal Command

Alternatively, if you like Terminal commands (for scripting or BetterTouchTool gestures), you can set scroll bars to always be shown with the following command:

defaults write -g AppleShowScrollBars -string Always


To reset this, you can use the following command:

defaults write -g AppleShowScrollBars -string WhenScrolling


You may have to restart the app (or Finder: killall -HUP Finder) for the change to take effect.

Per App Setting

As well as being set globally, these Terminal commands can be used to change the scroll bars specifically per app.

defaults write com.apple.Terminal AppleShowScrollBars -string Always


Replace com.apple.Terminal with the bundle identifier of the app that you want to change.