I've always used three questions to determine how we're doing:
1. Where are we?
2. Where did we plan to be?
3. Where should we be (based on what has transpired)?
The first 2 questions are easy. Traditional project management has used them
as their basis for years. If you weren't where you planned to be, then *it's
all your fault!*.
However, the 3rd question is the toughest. Given the things that we've
learned during the iteration, are we where we would expect to be, or are be
behind because we've been goofing off and spending all our time on Facebook?
Source: http://groups.yahoo.com/group/scrumdevelopment/message/51124