How we know she's thinking about running: It is Hillary Clinton. Also, a de facto campaign organization exists for her already. The Clinton team is pushing back on criticism of its non-profit. Hillary and Bill will campaign in Arkansas next year.
Will she actually run? Yes.
How we know he's thinking about running: Because "Cuomo" is Italian for "considering a White House bid." Andrew's father, Mario, was frequently mentioned as a potential candidate. And then there was BuzzFeed's look of a possible bid.
Will he actually run? It seems unlikely if Clinton runs.
How we know he's thinking about running: Our colleagues at the National Journal profiled him extensively in June. And people have been talking about him running for much longer.
Will he actually run? He will probably talk about it a little bit and then, if Hillary Clinton runs, not actually go through with it. After all, if you correctly identified O'Malley as the governor of Maryland, you are in the minority of Americans.
How we know he's thinking about running: He went to Iowa and, apparently, that's the only reason for anyone to go to Iowa. Oh, also he said he was exploring the idea.
Will he actually run? Probably. There's not much disincentive for the less-than-half-term senator to do so, and the qualities that (briefly) endeared him to voters in heavily-Democratic Massachusetts — his rugged good looks, his pickup truck — might endear him to the Midwest, too. Republican politicians form Massachusetts have won the nomination before.
How we know he's thinking about running: Last week, Christie vetoed a gun control measure he'd supported previously — a switch that will endear him to Republican primary voters. There is also the whole radical weight loss surgery thing.
Christie is currently running for reelection as governor of New Jersey. As The New York Times pointed out on Sunday, that campaign is also letting Christie establish a deeper fundraising base and giving him a chance to stay in the news.
Will he actually run? Yes. Christie, the "hottest politician in America," is a likely front-runner coming out of the gates. These things change, of course, but his brand of authoritarian individualism plays well.
How we know he's thinking about running: He released his birth certificate. He wanted to curtail growing questions about his eligibility by proving that he is a natural-born citizen — a prerequisite for the presidency, and not the sort of thing you do if you don't plan on running for president.
Will he actually run? Cruz is still new to Washington, but has gone out of his way to build an identity as a staunch conservative on Capitol Hill. A lot could change before the primary process gets underway, but Cruz has begun building the sort of identity that a 2016 candidate would want.