IDEA can show usages of the element at the caret with CTRL-SHIFT-F7. You can enable an option to do this automatically, under Settings → Editor → 'Highlight Usages of element at caret'.
You can then see all usages of the current identifier:
This feature has been extended in the Scala plugin to certain keywords.
We use this to show you the result expression if you select 'if', 'match' or 'case'
Choose 'for' to see the yielded expression.
Choose 'def' or 'return' for all the return expressions from a method:
Choose '=>' for the result expression of an anonymous function:
You can also select 'class', 'object', or 'trait' to highlight the statements that are part of the constructor, that is val/var definitions and other expressions. (This highlighting will require a nightly build > 26.03.2011 with IDEA 10.5 EAP)