code: private boolean isSymbolDuplicatedExternOnWindow(Symbol symbol) {
      Node node = symbol.getDeclarationNode();
      // Check that node is of type "window.foo";
      return !node.isIndexable()
          && node.isGetProp()
          && node.getFirstChild().isName()
          && node.getFirstChild().getString().equals("window");
    }
docs: Heuristic method to check whether symbol was created by DeclaredGlobalExternsOnWindow.java
  pass.