We know this because of things Reuters, the New York Times and the Washington Post reported in March 2011 and because of something then-Secretary of State Hillary Clinton said in her final testimony before the Senate Foreign Relations Committee in January.
Obama announced he had ordered the U.S. military to intervene in Libya's revolution on March 19, 2011, stressing that U.S. military involvement would come exclusively via air.
"We will not -- I repeat -- we will not deploy any U.S. troops on the ground," he said.
Ten days later, in an interview with NBC News, Obama suggested he might provide arms to the Libyan rebels. "I'm not ruling it out, but I'm also not ruling it in," he said.
That same day he told ABC News: "I think that it's fair to say that if we wanted to get weapons into Libya, we probably could."
Two days later, the New York Times, following up on a Reuters story, reported that Obama had already signed a finding authorizing covert aid to the Libyan rebels -- and noted that the administration had not actually started delivering arms to these rebels.
"Several weeks ago, President Obama signed a secret finding authorizing the CIA to provide arms and other support to Libyan rebels, American officials said Wednesday" the Times reported. "But weapons have not yet been shipped into Libya, as Obama administration officials debate the effects of giving them to the rebel groups."
The Washington Post chimed in with its own report: "President Obama has issued a secret finding that would authorize the CIA to carry out a clandestine effort to provide arms and other support to Libyan opposition groups."
The day before Obama indicated he was not ruling out arming the Libyan rebels, Adm. James Stavridis, commander of U.S. European Command, hinted at why this might not be a good idea.
"We have seen flickers in the intelligence of potential al-Qaida, Hezbollah," Adm. Stavridis told the Senate Armed Services Committee of the Libyan rebels. "We've seen different things. But at this point, I don't have details sufficient to say that there's a significant al-Qaida presence or any other terrorist presence in and among these folks. We'll continue to look at that very closely."
A Reuters reporter asked Secretary Clinton that day about Adm. Stavridis's testimony: "Is that part of the U.S. debate over any potential arms transfers to the transitional council?