American officials are adamant. The U.S. will respond - possibly with military force - if Iran crosses a red line and decides to actually make nuclear weapons.

But will the U.S. know with an degree of certainty that a line has been crossed?

The decision itself to push ahead really comes down to one person, according to Director of National Intelligence James Clapper. Clapper told a Senate hearing recently that any decision would be based on "the supreme leader´s world view and the extent to which he thinks that would benefit the state of Iran or, conversely, not benefit."

Clapper was referring to Ayatollah Ali Khameini, the supreme leader of Iran.

"It´s Khameini, period, full stop, end of sentence," agreed Kenneth Pollack, an Iran expert at the Brookings Institution.

But trying to read one person´s mind is no easy thing, observed Secretary of State Hillary Clinton recently, noting how Iraq´s Saddam Hussein was misread by U.S. intelligence.

"People sometimes say and do things that are at variance with what one might expect," the secretary of state said. "It´s still quite bewildering to me why Saddam Hussein wanted everyone to believe that he had chemical, biological and even nuclear weapons of mass destruction when apparently he did not."

Ephraim Asculai, a retired Israeli nuclear scientist, said there are three ways the world will know if Iran has decided to break out and make a dash to nuclear weapons: Iran tells everyone, the the International Atomic Energy Agency (IAEA) discovers it, or the intelligence community figures it out. But he is not terribly confident any intelligence service - whether it´s the United States´ or Israel´s or some other nation´s– will discover it.

"Depending on the intelligence community - this is not very good," Asculai said.

Even though the weaponization program may be stopped, Iran continues to push ahead with the more difficult component of a potential nuclear weapons program, the manufacturing of fissile material.

The IAEA is able to keep tabs on Iran´s uranium enrichment efforts. Although Iran is not believed to be enriching to weapons-grade, it is churning out stockpiles of lower-enriched uranium that could be rather quickly - within two to three months - enriched to the 90% needed for a weapon.

Current and former intelligence officials agree that IAEA inspectors play an instrumental role.

CIA Director David Petraeus recently told a Senate hearing: "I believe their past report was a very accurate reflection of reality, of the situation on the ground. I think that is the authoritative document when it comes to informing the public of all the countries in the world of the situation there."

By just about anyone´s account, Iran is one of the toughest intelligence targets.

Kenneth Pollack from Brookings, said the Iranians are "deliberately secret, but it´s also a very hard target because Iran is secretive by nature." He called the Iranian system "utterly byzantine and opaque" which will make it difficult to determine if the supreme leader, Ayatollah Ali Khamenei, has given the green light to produce nuclear weapons.

But former Defense Department official Colin Kahl said the intelligence community doesn´t have to hear the order from Khamenei.

"We would likely detect the effects of a decision by Khameini to go for a bomb even if we didn´t detect or intercept the order," Kahl said.

He also said there would be some telltale signs of Iran´s intentions.

"Convincing evidence the supreme leader had decided to for a bomb would include enrichment of uranium to weapons-grade level or a decision to kick out IAEA inspectors - both actions we would detect," Kahl said.

Without getting specific, U.S. intelligence officials have said they are using everything in the spy toolbox to figure out what Iran may be up to. There are numerous technologies and assets that are likely in play.

Satellites are taking snapshots of known and suspected facilities in Iran looking for signs of construction, movements and any other clues which suggest a weapons program.

Hyperspectral imagery from satellites as well as sensors placed near facilities are able to collect information along the electromagnetic spectrum to detect emissions coming from the building and compounds in the soil that could help determine if there are any radiological materials or other elements associated with nuclear weapons.

A CIA stealth unmanned aerial vehicle recently crashed inside Iran. And although the U.S. government said it was on a mission in Afghanistan near the Iran border, U.S. military officials told CNN the Sentinel drone was on a surveillance mission of suspected nuclear sites in the country.

Intercepting communications coming in and out of Iran falls under the purview of the National Security Agency.

Human intelligence is much tougher for the United States. The lack of diplomatic ties for more than 30 years means there has been no embassy for intelligence officers to use as cover.

But there are signs of human assets on the ground although it is unclear who they might be working for.

Over the past several years, five Iranian scientists associated with nuclear activities have been killed in similar fashion.

Who employed the assassin or assassins is unknown, although many people suspect the Israelis were behind the operations. The Israelis have been mum on any role they might have played.

The unexplained explosions at three sensitive Iranian facilities could suggest a possible sabotage campaign. And cyberexperts say someone with deep knowledge of Iran´s enrichment program contributed to the Stuxnet computer virus that disabled a number of Iran´s centrifuges. Again, who orchestrated the attack is not known.

Then there is the mysterious case in 2010 of the Iranian scientist who defected to the United States, seemed to have second thoughts and ended up returning to Iran. It´s unclear why Shahram Amiri went back to his country, but a U.S official said Amiri had received approximately $5 million for providing the U.S. government with "valuable, original information" on Iran´s nuclear program.

But it does raise the question whether there are other scientists in Iran working for the west.

Other intelligence services have also contributed to the knowledge about Iran according to the IAEA. Israel, Jordan, Saudi Arabia to name just a few have a vested interest in stopping a potential Iranian nuclear weapons program. Unlike their American counterparts, they have a better ability to blend into Iranian society.

The Iranian opposition group known as MeK, which took credit for publicly disclosing Iran´s then-secret nuclear facilities in 2002, is a question mark. The MeK is on the State Department´s terrorism list and many U.S. intelligence officials are skeptical about how much the group really knows. But it does have supporters in Iran and as one U.S. official put it, "It´s important to consider all leads in assessing Iran´s nuclear program and the MeK clearly tries to keep a close eye on it."

The Israelis reportedly work closely with the MeK.

Overall, former CIA Director Hayden does not consider the MeK "a critical part of the narrative" of Iran.

Open source information such as Iranian research papers and academic studies can provide potential clues to Iran´s nuclear knowledge.

And keeping an eye on commercial and industrial goods coming into the country not only gives insights into what Iran needs, but also opens the door to the opportunity to sabotage materials critical to a nuclear program.

But in the end, it comes down to whether all of those intelligence assets will paint an accurate enough picture of Iran´s activities to help President Barack Obama make what could arguably be the most critical decision of his presidency.

In a commentary on cnn.com last week, Hayden wrote, "The challenge for American intelligence now is to inform the president of an Iranian decision to weaponize its nuclear stockpile with sufficient confidence and in sufficient time for him to decide to launch a pre- emptive war in one of the world´s most sensitive and volatile regions." (© 2012 Cable News Network 03/26/12)