How can I find out if X follow Y on Twitter?

Question

What web application can I use to find if a user is following another one, on Twitter?

Update:

I found http://twitter-follow.appspot.com/ to be the one that suits my taste.

Warning: We've had flags that the above link is generating security alerts in Avast

Answer 1

Just plug each username into Does Follow and it will tell you.

Answer 2

http://friendorfollow.com/
http://isfollow.com/

Probably some more services out there to be found.

Answer 3

You can, of course, look on their Twitter profile to see who they are following. There is also a powerful webapp called Refollow which will allow you to do more powerful parsing and comparison of followers.