It won't hurt to read a little on the following topics
Clustering News Articles
Clustering Evaluation
The evaluation of clustering algorithms is a difficult problem—on the one hand, we can sort of tell what good clusters look like; on the other hand, if we really know that, we should label some instances and use a supervised classifier! Much has been written on this topic. One slideshow on the topic that is a good introduction to the challenges follows: http://www.cs.kent.edu/~jin/DM08/ClusterValidation.pdf.
In addition, a very comprehensive (although now a little dated) paper on this topic is here: http://web.itu.edu.tr/sgunduz/courses/verimaden/paper/validity_survey.pdf.
The scikit-learn package does implement a number of the metrics described...
