For the first economic impact payment (EIP1), which was sent to most taxpayers in early-to-mid 2020, you had to wait until the IRS sent you a letter with instructions on how to follow up with the missing payment.
If you have not received all or part of your second economic impact payment (EIP2) and chose to have your preparation fees paid from your refund when you filed your taxes in 2019, the IRS is working on getting your EIP2 directly deposited to your correct bank account.
All others that haven’t received EIP2 by January 15 should file their tax return and claim the Recovery Rebate Credit to have the missing amount added to their tax refund (or deducted from taxes owed). You can confirm whether or not you were issued the first or second payments by requesting your transcript on IRS.gov.
Not sure if the IRS sent you a payment at all? Use the Get Transcript tool on the IRS.gov website to request your account transcript and see whether a payment was distributed.
You can also claim the missing amount (as long as you qualify) with the Recovery Rebate Credit.