Ok so someone out of the blue has now started following you and even though you might not be that interested in their Twitter subject matter, out of politeness you follow back almost like returning the favour. Sometimes these phantom followers are on a mission to get as many followers as possible for what ever reason and as soon as you follow them they then un-follow you. May be its a status thing where it looks good having 500k followers and only following only 15. Anyway there is a way to spot if any of the people you are following are following back and here is how:
First step is to click on your normal twitter homepage and then move over to your right panel and click on the ‘following’ link as indicated below.
Now you should see a list of all the people you are following, now, to see if a particular person is following you back, click anywhere on their mini profile on left side so that it shows up on the right side panel. If you see an envelope icon (which incidentally is the direct message button) as indicated below then this means they are following you.
If this envelope/direct message is not present then this means they are not following you. Shame on you Lance Armstrong (see below).
These tips are based on the latest (as of June 2011) version of Twitter.