There's a portrait of George Washington in the Oval Office, but Joe Biden chose Thomas Jefferson and John Adams to decorate his office instead.

Why? Because Jefferson and Adams were the first two vice presidents who went on to become president in their own right.

"I joke to myself, I wonder what their portraits looked like when they were vice presidents," Biden recently told GQ magazine.

Is it even a secret that Biden truly hopes to succeed President Obama? Just about everyone assumes that Hillary Clinton is the front-runner for the Democratic presidential nomination in 2016, so it's an uphill climb for the vice president.

But watching what he's done over the last year or so is more than just a Beltway parlor game. It's a case study in how to tell when the people you've recruited to your team have started counting the days until you step down.

Here are three signs:

They make themselves invaluable.

In 2008, after he dropped out of the presidential nomination contest himself, Biden had to make a choice: endorse Obama or Clinton. He chose wisely, at a time when Obama still needed him.

Still, until recently it was basically a Washington joke to suggest Biden might succeed Obama. But then as GQ pointed out, over the last year or so...

... abruptly, Biden's stock started steeply rising, at least in the eyes of the public. Washington had been hyperventilating about the fiscal cliff, and Obama sent Biden in to broker a deal. Then came the killings in Newtown, Connecticut, and Obama sent Biden out to rally the public, Biden in to reason with Congress, Biden over to talk to the NRA. In 2013, Biden has emerged increasingly more visibly potent than his boss. THE MOST INFLUENTIAL VICE PRESIDENT IN HISTORY? one headline proffered.

They hustle to shore up weaknesses.

We've been hearing for more than two decades, since long before she held elected office, that Hillary Clinton has presidential aspirations. From First Lady to U.S. Senator to presidential candidate to Secretary of State--regardless of what you think of her politically, Clinton has done a masterful job of filling in the gaps in her experience.

Biden, for all his gaffe-prone goodness, has a deep government resume, including three decades in the Senate and service on both the judiciary and foreign affairs committees.

Moreover, Biden is in the right place, statistically speaking. Of the 17 U.S. presidents in the 20th Century, seven had previously been vice president. Four other vice presidents ran for president and lost (most recently, Al Gore in 2000).

They are open about it.

You can't be more obvious than Joe Biden. Even stepping beyond the political arena, the paths from COO to CEO in big companies, or executive officer to commander in military units are well-worn. Heck, the highest-paid CEO in America right now is a former COO, Apple's Tim Cook.

That said, as Marshall Goldsmith points out in Harvard Business Review, tacitly encouraging an underling's false hope to succeed the boss can be dangerous:

Succession charts aren't promises, [but] they are often communicated as such and can lead to frustration if they aren't realistic. Bottom line, don't jerk around high performing leaders with unrealistic development expectations. Only give the promise of succession if there is a realistic chance of it happening!

What do you think? Early as it is, does Biden hold out any hope of getting the Democratic nomination in 2016? And what other signs should you look for to see whether someone on your team is getting restless?