The amount shown with code W in box 12 of your W-2 is the amount contributed through your employer, whether or not is came out of your paycheck. Personal contributions are instead those that you yourself deposited into the HSA, not by payroll deduction through your employer. Such a deposit made by you personally would likely have come from your checking account, so if you don't remember making such a payment to your HSA, check your checking account statements for any such payments you yourself made.
