The easiest way to currently identify a blog that has do-follow links in their comments is the presence of an image that says “U Comment, I Follow”, like the one pictured below.
Now, the catch with these has historically been that there’s been no central repository of participants. Since it is an image, you cannot simply search Google for it either, as Google cannot read the text.

The Method
We discovered that a lot of the blogs that use a relatively static “alt” text. Now, while most search engines themselves do not search alt text in their standard results, image searches are entirely different matter.
By searching Google Image Search (or other image searches) for “U Comment I Follow” (with quotes is more accurate, but without quotes gives more results), we can pull a substantial list of do-follow blogs. By appending your given keyword(like “seo”), you can get more targeted blogs.
In addition, searching for “U Comment” or “I Follow” separately bring up a lot more results.

It’s More Accurate Than You Think
For whatever reason, most of the images Google returns for the above searches are not the images of the “U Comment, I Follow” project. However if you visit most of the sites listed, they actually DO have the image showing they’re a part of the project, Google just doesn’t return that as the search result. Odd, I know

Other Variations on the Method
Alt tags are not the only footprint available for this. It also uses relatively predictable image names.
You can search for these either using the inurl: command, or searching for the filename itself.

• ifollowblue.gif, ifollowgreen.gif, ifollowpink.gif, ifollowpurple.gif, ifollowltgreen.gif, ifolloworange.gif, ifollowwhite.gif,
• ifollowmagenta.gif
• ifollow.(gif/png/jpg, take your pick)
• utrackback_ifollow.gif
• ifollow-red.png
• inurl:ucomment
• inurl:ifollow

There’s a million more, so try and find them. They’re worthwhile if found within your niche. This entry is more of a tip than a complete guide.

source: slightlyshadyseo.com