Can\'t find one under
Xcode ▶ Preferences/Text Editing ▶ \"Code Folding Ribbon\" box.
Xcode 9
As it has already been commented, the code folding in Xcode 9 only works on entire functions. The closest I got to a way to figure out where does a curly brace closes (especially for long for or if blocks), is by double tapping in the curly brace. This will highlight that block of code until the curly brace closes.
Of course, this will work much better in Xcode 10 with the new code folding ribbon.