Thanks for the reply. Unfortunately, I closed my previous bank account so I have no way of tracking unless I contact them.
I checked https://my.uscis.gov/uscis-immigrant-fee/enter-data and entered my parents' information, and it seems like $220 might be due (see attached screenshot) for each of them. I am assuming if it was paid already, it wouldn't even show up, right? Also, it seems like if I had not paid the USCIS Immigrant fee, my parents should have received a handout with instructions on how to pay the USCIS Immigrant Fee (USCIS Immigrant Fee handout), per this guide, How Do I Pay the USCIS Immigrant Fee.