How can Google know that you paid to be in a directory?

Selected answers from the Dumb SEO Questions Facebook & G+ community.

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  Girish Kapoor: How can Google know that you paid to be in a directory and then devaluate the value of your link ?
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  Perry Bernard: I don't think it's hard for them to figure out which directories accept paid links. A directory that is eCommerce enabled is a bit of a giveaway.
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  Girish Kapoor: Thanks +Perry Bernard i am asking background process of Google. How it's know that you paid to be in a directory
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  Edwin Jonk: From the expert panel in this weeks SEO Questions hangout on air on 01:05:58 into the YouTube video: https://dumbseoquestions.com/q/how_can_google_know_that_you_paid_to_be_in_a_directory +Girish Kapoor
  
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  Perry Bernard: +Girish Kapoor ;well, let's put it this way: It doesn't "know" unless it can track your transactions linking to a listing, but if every listing on a certain directory has significant evidence to suggest the directory is selling the listings, it wouldn't be too far fetched to assume you also paid for yours. The assumption may be right or wrong, but the judgement will be based on probability and content profile where there is little doubt.