How do I go about figuring out which links to disavow?

Im taking over a business`s SEO because their last SEO didn`t communicate well. However, the last SEO`s approach was building a ton of links through submission sites with no content creation. Currently it looks like most of their links are not from legit websites, but they are ranking well in search... I want to look at all of their 3k+ backlinks and disavow some of the spammier ones. How do I go about figuring out which links to disavow? Does disavowing links even matter anymore, or does Google do this for you now? I don`t want to take away links that are positively impacting their rank. Any articles you can refer me to read would help. Thanks!