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Here are a few footprints which Google can use when evaluating your links (some are algorithmic and some manual if you are under audit/review and some are cross-referenced): Time of the link placement and any related groups of links generated within the same timeframe Link location (blog roll, footer vs content and navigation) Pattern of buying from known link sellers Strict use of exact match commercial terms in anchor text IP address / C-class Author name in article submission Open calls and invitations for link exchanges, selling, buying Widget anchor text manipulation Or anything else that can be seen as a link scheme.