Twitter in itself does not provide any way to find out if someone has unfollowed you. So here are some of the ways in which you can track and see when someone has unfollowed you on twitter. Most unfollower watchers are pretty simple to use.
Follow @goodbyebuddy on Twitter, this robot will automatically send you a direct message whenever someone unfollows you.
Follow @tless. It is another Twitter robot, which can keep track and tell you when someone unfollows you on twitter. It provides users with a daily summary with leads to a page where you can see the new followers and people who unfollowed you.
Nutshell Mail allows you to take control of your social networking accounts. It will send you a periodical newsletter which consists information such as followers gained, followers lost, latest tweets, direct messages and more.
Friend or Follow is a service which allows you to see which users on twitter are your friends, which are followers and who are not following you. Can be used to track loss of followers.