How can news sites show a candidate as the winner when less than 50% of the ballots have been counted?

Many major news sites (msnbc, cnn, fox) are showing that a particular candidate “won” even when there is a relatively narrow margin of victory and less than 50% of the ballots have been counted. How could the news sites make such announcements? What method do they use to tell whether a race has been “won” or is “too close to call”?