First, you verify the Amount Financed and payment schedule. If there are any fees, be sure the ones that are FCs were properly deducted from the principal to arrive at the AF. The payment schedule can be verified by amortization--but you must amortize by the US Rule method if that's how your note reads and the loans are serviced. If the ending balance is anything other than $0.00, then the disclosed payment schedule is wrong.
After determining that you are entering clean information, you can use APRWIN to generate an APR by the actuarial method. If the recalculated APR is within tolerance, you're done.
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...gone fishing.