How do I find out who claimed me on their tax return?

You cannot find that out; neither the IRS nor the state will tell you.

Just report the fact and file your return, and the IRS will straighten it out.

You can't. The IRS will not discuss another taxpayer's tax return with you, even a fraudulent one.The IRS won't tell you that. If you believe that no one was entitled to claim you as a dependent, file a paper income tax return and take your own exemption. The IRS will sort it out. So, You can only file your own tax return claiming yourself and let the IRS determine which return is correct.