Since we're now in the very early stages of the "invisible primary" the early jockeying for the nominations of both major parties in 2016 we're going to get a lot of speculative pieces about which candidates are running. See, for example, David Remnick's conviction that "Hillary Clinton is running for president."
Here's some guidance in how to talk about these candidacies.
The first distinction is one that, as the political scientist Josh Putnam has put it, there's a difference between candidates who are running for 2016 and those who will be running in 2016. In other words, several politicians might do what they need to do right now in order to run, but will for a variety of reasons choose to drop out between now and when they must formally enter the primaries and caucuses in the election year. (Why? Most likely some combination of how much they want to run and what their perceived chances are of actually becoming president). The conclusion to this is that a whole lot of politicians are currently running for president far more than will eventually contest the Iowa caucuses, although probably including most or all of those who will do so.
The second distinction is that what a candidate needs to do this year in order to be a real contender in 2016 depends on where the candidate is at right now. For lesser-known candidates, it might involve actively courting important party actors and donors, visiting the early states (not so much to appeal to voters there, but to secure "mentions" and therefore be taken more seriously by party actors), perhaps rolling out policy ideas, and more.
On the other hand, for a true heavyweight candidate and Hillary Clinton certainly qualifies all that might be required right now is to avoid answering questions about a potential run in a way that anyone would actually believe. She'll have plenty of time to gather resources in 2015 for a 2016 run, so there's little she needs to do now other than to signal to those who already want to support her that she may well be running later in the cycle. In this sense, Remnick's conclusion is absolutely correct: Clinton is, right now, running for president. Whatever she may or may not be doing a year or two down the line.
However, there's a major caveat to any of this. Signaling a run has very little downside or costs for a candidate in her position. Therefore, the odds are pretty good Clinton would act as she has, even if her true intension is to have nothing to do with the nomination contest.
None of us can get inside the heads of any politician Hillary Clinton included and determine their true thoughts about their careers. All we can do is observe their actions. And for Clinton, we could expect her to be acting exactly the same right now no matter what her real intentions might be. So all we can really say is that she's doing everything she needs to do now if she intends to be running in 2016, and to wait and see what happens next.