How to use IntelliJ IDEA to find all unused code?

In newer versions of IntelliJ it's under Code -> Analyze Code -> Run Inspection by Name.

In IntelliJ versions before that, you should run it from Analyze -> Run Inspection By Name:

Than, pick Unused declaration:

And finally, uncheck the Include test sources:

After you've run the Inspect by Name, select all the locations, and make use of the Apply quick fixes to all the problems drop-down, and use either (or both) of Delete unused parameter(s) and Safe Delete.

Don't forget to hit Do Refactor afterwards.

Then you'll need to run another analysis, as the refactored code will no doubt reveal more unused declarations.

Just use Analyze | Inspect Code with appropriate inspection enabled (Unused declaration under Declaration redundancy group).

Using IntelliJ 11 CE you can now "Analyze | Run Inspection by Name ... | Unused declaration"