If you have copies of your prior year tax returns, you can check them to see how many times you claimed the credit. If you started college in 2011 and were a student for four years in a row, from Fall Semester of 2011 to Spring Semester of 2015, it is possible that you claimed the American Opportunity Credit in 2011, 2012, 2013 and 2014.
If you don't have copies of your prior year tax returns, but you prepared them using TurboTax, you can unlock access to those returns by purchasing PLUS.
https://ttlc.intuit.com/questions/2758553-how-to-unlock-prior-year-returns-through-turbotax-online
If you didn't use TurboTax, you can request transcripts of these returns from the IRS.
https://www.irs.gov/uac/Newsroom/Request-a-Transcript-or-Copy-of-a-Prior-Year-Tax-Return
In any case, you would see an entries on line 50 &68 of your 1040 or lines 33 &44 of your 1040-A for years that you claimed the credit.
However, if your parents claimed you as a dependent during any of those years, the credit should have been claimed on their tax return, not yours.
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