Usually I get the Idea from some products and then do some google to see how fair it is to write about that as i dont want to compete with thousands of high rank sites. If there are not much items about them then i write abt that product. NOTE: Using this got me Page 1 within 1.5 month of my website launch in Google.
there are so many in the net. just surf... there are lots of those and you need only to analyze better.
somewhat true .... you can try this one to be more accurate http://tools.seobook.com/general/keyword-information/