Give comment on blog that doesn't have nofollow attribute can give backlink that will be indexed by Google. A lot of backlink is one factor to increase page rank. Are there any way to know that comment system on the blog have nofollow or not?
The easiest way is by seeing the source code and find if there is rel="nofollow" or rel="external nofollow the code. But this is not practical way because you must view source code on each site and find nofollow attribute.
The other way by visiting dofollow directory. Allmost all blogs on the directory have erased nofollow attribute. But some blog will active again the attribute.
There is another practical way. This way using with firefox browsing with SearchStatus extention.
After install, restart your browser and you will see @ icon on the status bar. Right click on the icon and click "Highlight Nofollow Links" like on the picture below.
After that you can see that all link with nofollow attribute will be highlighted.
This way is simple, just type blog address, wait for loading and you can see link with nofollow attribute. Except that you can also check page rank or alexa rank of the blog or site that you access.
Have a nice try!