If you are not sure, you likely did not make any estimates and should report $0. Estimates are in addition to withholding on your paycheck. @baldietax is correct- you can log into your EFTPS or get a transcript of your tax data from the IRS.
Here is some helpful information on obtaining a transcript: What is an IRS Tax Transcript and How Do I Request One? You are going to request a "tax account transcript" since the tax return has not yet been filed.
