Hello dear, I will suggest you to use the tools for checking dofollow and nofollow backlink. You can easily check dofollow and nofollow by this tools. I use this tools. If this link show blue it is dofollow link.This link shows red, it is nofollow link.

Nodofollow Checker

Its easy select the link and right click on it. Then go to the 'inspect element'. It will show you the code and within it if you see 'rel="no-follow"', then its a no-follow or otherwise its a do-follow.

Do-follow is good but I think to SEO friendly must both do-follow and no-follow

Right click on the link and check for inspect element and check whether its no follow or do follow
Example of Nofollow Link:

<a href=”http://www.google.com/” rel=”nofollow”>Google</a>

Example of Dofollow Link:

<a href=”http://www.google.com/”>Google</a>

Note: By default all the hyperlinks are dofollow. So, you don’t need to do anything to make a link do-follow.

Install a moz toolbar on your web browser. It can highlight all the dofollow and nofollow links on a site.

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You can use seoquake, It will highlights all links that has rel=nofollow attribute.

you can use SEO do follow and no follow tool for this problem i think it best solution for you.

Don't put to much emphasis on Dofollow Links, just focus on, commenting, no matter nofollow or dofollow, it will give your benefits.

Btw, if you are really looking to get commenting blogs, then this random post from Google, might help you:- Latest List of 500+ High PR Dofollow commentluv Enabled Blogs - OnlyLoudest

You can use MOZ toolbar for google chrome to check no-follow or do-follow link.

you can set up addon to find blog or forum nofollow
NoDoFollow Add-ons for Firefox