For small to medium changes, by reading the XML. Everything is named sanely, and the XML hierarchy matches the view hierarchy. Constraints can be a little hard to understand, because they use opaque identifiers, but it's possible. I'll occasionally have to open the storyboard/NIB in Interface Builder to confirm/disprove something.

For large changes, it can be unmanageable. But large changes are hard to code review anyway, so we try to avoid those.