1. Google Blog says it's official in the title. This blog says it's "not affiliated with Google".
2. Google's official blogs are written by more than one person. Here, there's only one person who does that.
3. Google's blogs use possessive to show their origin: "Googler insights into product and technology news and our culture".
4. Google's blogs are copyrighted by Google. This blog uses Creative Commons license.
5. Google's blogs don't show ads. This blog used to do that.
6. Every Google blog lists all the official Google blogs.
7. Google's blogs talk only about Google services and products. This blog also talks about competitor's products.
8. Google's blogs don't accept comments. This blog does.
9. Google's blogs don't have posts about yet to be released features or personal opinions about Google products. They don't start a post with: "The latest version of Google Toolbar for IE has a strange new feature: keep Google as the default search engine."
10. Usually Google's official blogs don't talk about a Google Operating System. Basically because it doesn't exist.
Update: If this post seems confusing, a post and some discussion from Google Blogoscoped might be helpful. Also read this disclaimer.
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