There are three pretty good ways to find out if someone unfollows you on twitter.

1. @unfollowr is a tool which allows to keep track of people un-following you on twitter. Once somebody stops following you, you will receive a direct message notification. To start using the service, just follow @unfollowr. Link http://oneforty.com/item/unfollowr

2. Qwitter e-mails you when someone stops following you on Twitter and tells you what your last tweet was before they stopped. Link http://oneforty.com/item/qwitter Inactive since writing this.

3. Goodbye, Buddy is an Unfollower monitor with funny direct messages and website with unfollower history for each tracked twitter account. goodbyebuddy.com/your-twitter-name. Link http://oneforty.com/item/goodbye-buddy

4. Chirpstats (formerly Twitterless) Just like Goodbye, Buddy. Chirpstats helps you keep track and learn more about your follower-base. Simply follow @chirpstats on Twitter and it will automatically track your followers.

5. Who.Unfollowed.Me. Who.Unfollowed.Me is a web-based Twitter application that informs you who unfollow you, in real time, without waiting for a direct message or email.