How Google Detects Paid Links?

We consider that Google finds out about paid links with the help of its amazingly huge statistics. Google has statistics of the whole Internet and it perfectly knows:

Average NATURAL link building speed (speed of new links appearing to a website).
Average NATURAL mix of different Google PageRanks of backlinks.

We think that:

If a link building speed is TOO QUICKLY, Google algorithm detects that and treats these new backlinks as BOUGHT.
If a mix of Google PageRanks of built backlinks is NOT NATURAL, Google algorithm detects that and treats these new backlinks as BOUGHT.

What Mix of Google PageRanks is NATURAL?

With the help of our Backlink Checker Tool we have collected and analyzed backlinks of more than 300,000 websites. Our statistics have shown that NATURAL mix of Google PageRanks is the following:

Google PR	% of Backlinks
PR0	~69.6%
PR1	~9%
PR2	~7%
PR3	~6%
PR4	~4%
PR5	~2%
PR6	~1%
PR7	~0.8%
PR8	~0.5%
PR9	~0.09%
PR10	~0.01%

This means that you need to build or buy links of different PageRanks to your website. This also means that you should NOT buy only high PR links because Google will see that your site received only high PR links and no low PR links and will consider those links as BOUGHT!

Which Link Building Speed is NATURAL?

This speed is different for each website because of different domain age, link popularity, and other factors. We consider all of them in our link building campaigns, but from our 5 years experience of website promotion natural average link building speed is ~ 600 backlinks per month (note: ~70% of them should be PR0).