Inspection of the provided WebCore binaries and the libraries that use them, Objective-C runtime information (often via Cycript; http://cycript.org/), and the fact they include headers (but not implementations) in the iOS releases.
Or maybe there is some common code within WebCore that is shared between Mac and iOS, and one of the files needs to import the WAK headers to compile correctly?
While it is fun to get worked up into a frenzy, I doubt this is an LGPL violation.
As static linking is covered by the LGPL, the source code for that (and other) classes must be made available, yet they are not. How is that not a LGPL violation?
(You can also see via the "class-dump" utility that there are many more WAK classes inside the WebCore binary. No source code has ever been released for those classes.)