T-mobile USA IMEI checker: https://www.t-mobile.com/resources/bring-your-own-phone
Cricket Wireless IMEI checker*: https://www.cricketwireless.com/cell-phones/bring-your-phone
Verizon Wireless IMEI checker**: https://www.verizon.com/od/prepaid/bring-your-own-device/#/deviceDetails
*AT&T no longer has an IMEI checker on its website, but they own Cricket Wireless, which does provide one, and uses AT&T’s network.
**For Verizon, you can skip the make and model info; then choose “I’m not sure” for the question about where you purchased the device.
This also applies to all the MVNOs that run on the above major networks.
EDIT: Linked this to: https://source.puri.sm/Librem5/community-wiki/-/wikis/Cellular-Providers