How can I tell whether or not my FileVault recovery key is stored in iCloud?
Assuming that your disk is APFS formatted, run the following command in the Terminal:

diskutil apfs listusers /


Then you need to check if it lists "Type: iCloud Recovery User" and "iCloud Recovery External Key" - if that is the case, then your computer thinks the recovery key is stored in iCloud.

If you're worried that Apple somehow lost your key and need to verify that they still have it, you'll have to go through the actual recovery process to try unlocking your drive without your passphrase. That is the best test to see if everything is setup and working as it should, and that Apple has your key.