That requires figuring out what each prospective candidate would be doing if she were in fact running. Sometimes that’s easy; visits to Iowa and New Hampshire are the traditional obvious giveaways. Sometimes it’s high-profile speeches. Sometimes it’s a book; that was the tip-off that Rick Perry was running in the 2012 cycle despite multiple denials from the Texas governor. The point here is that it may differ depending on the nomination battle context (whether or not there is a strong frontrunner, for example) and the prospective candidate’s situation — whether she already has national name recognition, whether she needs more of an introduction to important party actors and plenty more.
At any rate: The key to all this is that the penalty for denying a White House run and then making one is approximately zero. And there may be perceived penalties for saying “I don’t know” or “yes,” even for candidates who are definitely running as of now. For example: Prematurely declaring a run is a good way of raising expectations, which might be good for relatively obscure candidates, but is often not what higher-profile candidates want. Or politicians just might find answering the question constantly an annoying distraction, and a clear, firm “no” is the best way to duck some of that. At least until the next candidate-like action, when it’s bound to start up again.
At this point, there’s an excellent chance that Warren herself has no idea whether she’ll be an active candidate in 2016. One way or another, she has very little to lose from denying it for now. They’ll be plenty of time for figuring out what she’s up to down the road.