So what is a "dofollow" blog/site & how can you identify one?
A few years ago the Big 3 search engines decided to support a rel="nofollow" attribute for the anchor tag that publishers can use to prevent external links on their website from gaining from their reputation. This kind of blocking is meant to discourage comment spammers from injecting their links onto popular websites (which enjoy a good search engine ranking) & thus resorting to Spamdexing.
There is no rel="dofollow" attribute defined as such and any blog or website that does not enforce rel="nofollow" attribute on external links posted on it can be considered a "dofollow" site.
One way to identify such a website is by using the Nofollow bookmarklet created by John Mueller, a Googler. You can drag and drop the Nofollow bookmarklet on this page to your bookmarks bar and then hit it whenever you want to see links with the rel=nofollow property on any web page. This bookmarklet inserts a tiny bit of CSS into the top of the page you're currently viewing to highlight nofollow links. If the links within the comments on that page are not highlighted, its possible that the site allows its "link juice" to flow.
The bookmarklet works in Firefox, Opera, Safari & Chrome but not IE8. Following up on the error message "Unexpected call to method or property access" emitted in IE8 led me to a thread on StackOverflow. I have adapted the original bookmarklet code based on a resolution mentioned there to make it work in IE8 as well. Here's the modified code -
You can get the modified bookmarklet for IE8 from here.