Yes, it will usually be known before the final count.
The faithless elector from Minnesota in 2004 was known on the day of the voting as the State's Secretary of State counted the votes and announced the vote tally.
One of Minnesota's 10 presidential electors broke from the pack and cast a vote Monday for John Edwards, the Democratic vice presidential running mate for John Kerry.
Once all votes were in, Secretary of State Mary Kiffmeyer and an aide pulled them out, counted them and announced the total. A tally sheet was sent to Congress, which announces nationwide totals in January.
Politico reported in 2008 that "there were no reports of faithless electors Monday".
Fox News reported the voting procedure in some states in 2012.
About the voting
The electoral college voting is organised by the respective states and some states allows the public to watch them vote.
May I attend the meeting of my State’s Electors to watch them vote?
Generally, each state’s Electors vote at their respective state capitols. Each state determines whether or not the voting is open to the public. To find out if your state’s meeting of Electors is open to the public and if so, what the process is to view the vote, contact your Governor’s Office or your Secretary of State.
Whether or not the vote tally will be known depends on the particular state. The Secretary of State may choose to announce the vote tally while certifying the vote.
However, usually faithless electors will be reported on the day of the voting (Dec 19) since news organisations will be following the voting in each state. I believe that there will be more hype this year due to reports of potential faithless electors.