Although Firefox NoDoFollow plugin is a quick way to check the nature of link but don't skip to build links from even nofollow sites as they may help in bring the traffic to your site.
I've heard Google is monitoring these baclinks method and will no more give any priority to them. Only natural backlinks will be considered in SEO.
To find whether the link is dofollow or nofollow you either look into the coding or for google chrome install plugin called SEO for chrome and select the option of show nofollow and nofollow links will be hi lighted with dashed rectangle