Google Disavow Tool lets Google Search Bots ignore spammy or negative baclinks to your site.This is an advanced feature and should only be used with caution. If used incorrectly, this feature can potentially harm your site's performance in Google's search results. We recommend that you only disavow backlinks if you believe that there are a considerable number of spammy, artificial, or low-quality links pointing to your site, and if you are confident that the links are causing issues for you .
How to find out Spammy Backlinks to your site :
You can find backlinks to your site using many tools available online, while some are free and some are premium like Ahrefs Site Explorer & Backlink Checker which is most popular tool , also Moz's Open Site Explorer or Even from Google Webmaster Tools Links to Your Site Option.
Here is What Matt Cuts Says :
Here we will show you how to find backlinks using Google Webmaster Tools :
1. Login to your Google Webmaster Tool Here : http://google.com/webmasters
2. Select the site from Dropdown List which you want to analyse backlinks.
3. Now Select Links to Site From Right Sidebar as shown below :