Unused String and Image Resources in Android

In Android Studio (tried up to Version 2.1.3), you can simply go to Main Menu -> Analyze -> Run Inspection by Name..., and select Unused resources

(It runs Android Lint for you.)

In Android Studio there is a shortcut available.

Press Ctlr + Alt + Shift + i (or Cmd + Option + Shift + i on Mac)

This will open a dialog in which you can type "Unused" to see a number of options.

I generally type "unused resources" and get a list including layouts, strings, drawables, etc.

In Android Studio 2.0 and above

in menu select Refactor-->click on Remove Unused Resources...

Improving @Ramesh R answer, the best way is Right click on your res folder, Select Refactor and Remove Unused Resources.

This tool is fantastic, my applauses to his developer xD;

you can use lint

Android Lint is a new tool for ADT 16 (and Tools 16) which scans Android project sources for potential bugs. 

Here are some examples of the types of errors that it looks for:
Missing translations (and unused translations)
Layout performance problems (all the issues the old layoutopt tool used to find, and more)
Unused resources
Inconsistent array sizes (when arrays are defined in multiple configurations)
Accessibility and internationalization problems (hardcoded strings, missing contentDescription, etc)
Icon problems (like missing densities, duplicate icons, wrong sizes, etc)
Usability problems (like not specifying an input type on a text field)
Manifest errors and many more.