Not that I would expect Indiana to swing blue here, but with 8% of polls reporting, Donald Trump has already clinched Indiana.
How does this happen?
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Journalists are allowed to write what they want. =)
In practice, it is often reasonable to assume that the state overall results will have a similar distribution to the portion which is in. You can do hypothesis-testing to determine the likelihood of the vote reversing further down the line. Once it goes low enough, the journalists will call the election over.
In reality, the state will continue counting ballots dutifully until the results are certifiable. However, national attention will go elsewhere, away from this boring state.
As an addendum, since this question is getting interest again (curiously, 4 years later): While I claim that we can make assumptions about the state overall results, that's really only true if one can consider the sample to be homogeneous enough that your sample can be considered Indepentent Identically Distributed (IID), a key statistical criterion. For many states this is not quite true, especially since rural voters often vote different than urban voters. One may have to look at much more fine grained results, such as district-by-district results and only assume behaviors such as IID at that level. This can have interesting implications when some districts have a more complete vote tally than others. Some districts may be completely tallied, with a relatively definitive result, while others may still be a tossup with very few votes counted.
It's still hypothesis-testing, it's just a much more complex distribution to do the testing on.
From what I have read in 2020, some analysts are also seeking to account for early voting vs. at-the-polls voting. They are arguing that the distributions for the demographics of these two groups may indeed be different. Perhaps in 4 years this question will have renewed interest again and I can update the answer with what we learned!
They know from which counties the votes are coming from, so one can estimate if Clinton can win the state by getting a lot of votes from counties that tend to vote Democratic or if Trump doesn't get the votes he expects to get from counties that vote Republican. Now, if the voting results so far have validated the exit polling and if the exit poll results tell you that there aren't enough Clinton votes for Clinton to win given how the voting is going so far, then the race will be called for Trump.