What is your methodology for defining whether a review that does not include a star rating or other objective grading scale counts as a Fresh or Rotten rating on the tomatometer?
Most critics from the Online Film Critics Society (OFCS) enter their own quotes and ratings. For critics who don't enter in their own quotes and ratings, it's basically up to the judgment of the editors. They take into account word choice, rating (if any), tone, and who's the critic in their determination of whether a review is positive or not. If an editor is not certain about a review, it is sent to another editor for a second opinion. "Wishy-washy" reviews, reviews that are really difficult to determine if the critic recommends the film or not, are usually given a Rotten because if the critic is not confident enough to give the movie even an implied recommendation, then we shouldn't either.

On the Blade 2 reviews page, you have a negative review from James Berardinelli (2.5/4 stars), and a positive review from Eric Lurio (2.5/5). Why is Berardinelli's review labeled Rotten and Lurio's review labeled Fresh?
You're seeing this discrepancy because star systems are not consistent between critics. For critics like Roger Ebert and James Berardinelli, 2.5 stars or lower out of 4 stars is always negative. For other critics, 2.5 stars can either be positive or negative. Even though Eric Lurio uses a 5 star system, his grading is very relaxed. So, 2 stars can be positive. Also, there's always the possibility of the webmaster or critic putting the wrong rating on a review.