Super PACs must disclose their donors.
Super PAC is a term widely used to describe independent-expenditure-only PACs that were started in response to Citizens United.
Citzens United, held that limitations on corporate independent expenditures were unconstitutional, but also that "the disclaimer and disclosure requirements [for those independent expenditures] are valid" (Citizens United 2010, Bauerly and Hallstrom 2012).
With the advent of the Internet, prompt disclosure of expenditures can provide shareholders and citizens with the information needed to hold corporations and elected officials accountable for their positions and supporters.
One site that lets you easily browse Super PAC donors is OpenSecrets.org. They mention:
Super PACs must [...] report their donors to the Federal Election Commission on a monthly or quarterly basis -- the Super PAC's choice -- as a traditional PAC would.
Some donors donate via an LLC, which makes it more difficult to trace a donation back to an individual. (Potter, 2012) For example, HFNWA LLC donated $1,000,000 to the Senate Majority PAC. It is possible to connect that donation back to Franklin L. Haney, but only through additional investigation. (Fenton 2014, HFNWA LLC Registration)
I recommend reading (Bauerly and Hallstrom 2012) for more details and for reasons why disclosure doesn't always result in transparency. Bauerly is the chairperson of the Federal Elections Commission and Hallstrom is her chief counsel.
Bauerly, Cynthia L., and Eric C. Hallstrom. Square Pegs: The Challenges for Existing Federal Campaign Finance Disclosure Laws in the Age of the Super PAC. NYUJ Legis. & Pub. Pol'y 15 (2012): 329.
Citizens United v. Federal Election Commission, 558 U.S. 2010 (Supreme Court 2010).
Fenton, Jacob. Corporate donors still prefer the shadows. Sunlight Foundation. 2014.
HFNWA LLC Registration. Arkansas Secretary of State. Filed 2012.
Potter, Trevor. Five myths about super PACs. The Washington Post. 2012.