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Google has a well-trained paid-link tracker who uses some well known features of paid links making them easy to spot. See how I would spot paid links if I were Google. But Google can do it better and faster as they have the firepower.
Google knows more then we imagine and more then we want it to. Any scheme, any blackhat method, and trick is known but Google reserves the right whether to punish or not the offender. And every decision it makes is pragmatic and not based on intuition but evindence.