can anyone explain me how we know it is do follow or no follow ? thanks in advance
You can find the dofollow or nofollow tag by using some tools as well. Also you can check in view page source of any page search the URL and check if there is any nofollow or external nofollow code exist if yes then it is nofollow if not then it is dofollow. 
Either it contains rel=“nofollow” or it doesn’t. Just view the source to see whether that’s there or not.
You can view thru the page source which says see if the link is no follow or do follow, or you can use seo tools for free like seo tool for mozilla or chrome - the link is highlighted with blue or red if it is do follow or not.
to check this you can use NoDoFollow adons in mozilla browser it’s really very useful and i am using it as well.
You can use different firefox addon like seo4firefox to check a site dofollow or nofollow.If the site is no follow then it will color that in red or if it do follow then you will not get any change.
Manually, we check no follow by the source code. But, it is easy to check the links which have no follow just by installing the add on Search Status on firefox. This, tools highlights the no follow link in red color. Hope, it helps every one because it consumes our time.