I'm shocked that it's so hard to find the answer to this question:
How can I see which people following me also follow [some other person]?
Twitter uses this precise relationship to determine who gets to see a tweet I send to someone. The @user address form goes to the recipient and anyone who follows both of us.
How am I supposed to know who's following both of us? I can see a list of who is following me but I shouldn't have to go to every person's profile to see if they're also following person X.
Tell me twitter didn't implement a piece of recipient logic that fundamental and powerful, without allowing users to see the list of people their tweet is going to!