there are two ways:
1) did you receive Letter 6419 from the IRS stating what you received (and if filing joint there were two letters that needed to be added together)
2) review your bank account statements from July - Dec, 2021. They payments were deposited for that 6 month period right around the 15th of the month.
the payments should have been $300 per month for each child under the age of 6 plus $250 per month for each child age 6-17 (based on the children listed on your 2020 tax return, but using their ages as of 12/31/21).