How can i find out for certain website that which keyword they have been ranked. for example www.trellian.com is ranked 6/10, but i am not sure which keywords is ranked 6/10. thanks
seodigger was a good tool for this You can still see some data on semrush.com. I would recommend that you use google webmaster tools to see some of your ranks. BTW your 'pagerank' is 6. It's not same as your rank for any keyword. You will rank 1 for trellian or 'trellian software' but you will be no where for keyword 'movie reviews' (just an example)
6/10 is a PR - It shows the Importance of the Page. SERP - Search Engine Result Position - It shows the your website position for particular keyword. Don't confuse it.