Analyzing Twitter Relationships I'm not even sure if this is the right StackExchange to post this, but it seems like sentiment analysis would go here.

What would be the best approach to determine if two people on Twitter are actually friends?

I think that the first criteria would be that they are following each other. This would eliminate all of the celebrities, companies, etc. that a person follows.

The second criteria could be physical proximity to one another. If two people tweet from the same general areas, then there's probably a higher chance that they actually know each other.

After that, I'm not sure what criteria I should look for. Would sentiment analysis of their tweets to each other be a viable option? With that friendly sentiment could indicate that they actually know each other.
My metric of choice would be the communication by direct message. However, this metric is hard to come by since measuring it requires the user's consent.

I do not think that using similar hashtags or terms has any say over friendship. People tend to adjust to the language they read and do not require personal bonding to do so.

I think mentions are a good way to identify friends and foes alike. A good way would probably be to see whether mentions beget a reply. Users tend to 'reward' pleasant and useful mentions with replies. However, the semantics of these messages are very important and only quantified analysis may not be enough.

Since I cannot comment yet, I would like to reply to the suggestion of using machine learning. I do not think that machine learning can effectively identify friends, even if one manages to get 'sufficient' data. Using Twitter is a rather creative process and generally not uniform across social circles.