* Manually: in a web browser, if you right click and select "Inspect" you can reach the source code of the link. There you can review the HTML element and view it’s properties. If you look at the properties and don’t see rel="nofollow" in the source code then the link is a dofollow link.
* Automated: using Browser extensions it's a lot more convenient. E.g. the LRT Link Checker extension for Chrome or Firefox gives you the option to review that source code on mouse-over with your cursor, and also highlight links in different colors, depending on their type.
* Even more automated: a link analysis software like LinkResearchTools allows you to do mass evaluation of links for Nofollow or (Do)follow.