Use Homebrew to detect if X11 is installed.

Library/Homebrew/cask/lib/hbc/installer.rb

@@ -179,7 +179,7 @@ module Hbc
 
     def x11_dependencies
       return unless @cask.depends_on.x11
-      raise CaskX11DependencyError, @cask.token if Hbc.x11_libpng.select(&:exist?).empty?
+      raise CaskX11DependencyError, @cask.token unless MacOS::X11.installed?
     end
 
     def formula_dependencies

Library/Homebrew/cask/lib/hbc/locations.rb

@@ -171,14 +171,6 @@ module Hbc
       def pre_mavericks_accessibility_dotfile
         @pre_mavericks_accessibility_dotfile ||= Pathname.new("/private/var/db/.AccessibilityAPIEnabled")
       end
-
-      def x11_executable
-        @x11_executable ||= Pathname.new("/usr/X11/bin/X")
-      end
-
-      def x11_libpng
-        @x11_libpng ||= [Pathname.new("/opt/X11/lib/libpng.dylib"), Pathname.new("/usr/X11/lib/libpng.dylib")]
-      end
     end
   end
 end

Library/Homebrew/cask/test/cask/depends_on_test.rb

@@ -93,6 +93,7 @@ describe "Satisfy Dependencies and Requirements" do
   describe "depends_on x11" do
     it "succeeds when depends_on x11 is satisfied" do
       x11_cask = Hbc.load("with-depends-on-x11")
+      MacOS::X11.stubs(:installed?).returns(true)
       shutup do
         Hbc::Installer.new(x11_cask).install
       end
@@ -100,7 +101,7 @@ describe "Satisfy Dependencies and Requirements" do
 
     it "raises an exception when depends_on x11 is not satisfied" do
       x11_cask = Hbc.load("with-depends-on-x11")
-      Hbc.stubs(:x11_libpng).returns([Pathname.new("/usr/path/does/not/exist")])
+      MacOS::X11.stubs(:installed?).returns(false)
       lambda {
         shutup do
           Hbc::Installer.new(x11_cask).install
@@ -110,7 +111,7 @@ describe "Satisfy Dependencies and Requirements" do
 
     it "never raises when depends_on x11: false" do
       x11_cask = Hbc.load("with-depends-on-x11-false")
-      Hbc.stubs(:x11_executable).returns(Pathname.new("/usr/path/does/not/exist"))
+      MacOS::X11.stubs(:installed?).returns(false)
       lambda do
         shutup do
           Hbc::Installer.new(x11_cask).install