How a search engine finds a paid link

How a search engine finds a paid link

We know that paid linking is spamming, how a search distinguish b/w paid and free linking? What is paid linking exactly or how to handle all this

Google and most other search engines use links to determine reputation. Link-based analysis is an extremely useful way of measuring a site's value, and has greatly improved the quality of web search. A site's ranking in Google search results is partly based on analysis of those sites that link to it. Both the quantity and, more importantly, the quality of links count towards this rating, that it is free or paid links.