Estimated tax payments are made quarterly and you should know if you made them because you would have sent 4 quarterly checks for prepayment of taxes owed to the IRS and/or your state. These payments are generally made by persons who don't have regular withholding through an employer on their paychecks (i.e. a self-employed person). But, sometimes people who plan on making large sales of assets or other items make these payments to ease their tax burden at the end of the year.
Here is a link for general information on estimated tax payments - but it seems unlikely that you would not remember making these payments
https://www.irs.gov/businesses/small-businesses-self-employed/estimated-taxes
Note: If you did make estimated tax payments, but you just can't remember the amount of the payments - then, you would want to check with your bank records for copies of the payments.
