How do biologists estimate that there is 8% of protein coding genes in human genome?
S/he's off by about 6 - 7%.

As per the national human genome research institute approximately 1% of the human genome encodes proteins. We can tell this by looking at the sequence (which we're now able to do quite quickly) and identifying those portions of the sequence can be translated to protein using the genetic code.


  Scientists have been able to identify approximately 21,000 protein-coding genes, in large part by using the long-ago established genetic code. But these protein-coding regions make up only approximately 1 percent of the human genome...