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

Statistics. Of course they must put quotes around “won” or state that it’s an approximation, but they can estimate probabilities with impressive accuracy. I suppose they can determine whether an election is “too close to call” when the difference of the probabilities between two candidates meets a certain threshold. I have nothing to base this information on though.

I used to work for Gorden Black polsters. They go by another name now. But we did exit polls and they were fairly accurate indicators of how people voted. We did them all day from the time the polls opened and by the time the polls closed we already knew who had won.

Exit polling is the way this is done. The pollsters have all kinds of experience with various polling places, and they know which ones to pick, or how to pick a random sample of polls so they can get data that will allow them to make accurate predictions.

Back when Shrub was first elected, the pollsters got it way wrong, and so they had to go back to the drawing boards and figure out why they made the wrong predictions based on exit polls. Similarly, tracking polls sometimes get it wrong, and they also go back to the drawing boards, analyze the problem, and fix it.

Polls are become more an more accurate, so the news folks can “call” an election before the votes have been counted and, indeed, if they wanted to, before the poll have closed.

Apart from exit polls, it also involves knowledge of which precincts have reported, how many voters they contain, in which party (if any) those voters are registered, which way those districts usually swing, among other things. Back in the ‘80s before exit polls really took off, my news editor used to call dozens of races as soon as we got the first results, and media across the state followed him because he had such thorough knowledge of this minutia. As @wundayatta notes, these systems are not flawless but they work most of the time.