Similar Article Search

It seems some blog networks do not limit their search to Copyscape looking for straight duplicate content, but also search for similar articles that pass Copyscape but are contextually similar. Does anyone know how these searches are conducted? I've only been able to find services that allow you to compare two URLs for similarity, but none that search the web in its entirety.

Thanks in advance...

I prefer Google, as it is the best source for finding the uniqueness of an article. Copyscape can be fooled because there are flaws in its algo but the same can't be done with Google. Just like you, I haven't found better options for copyscape and Google.