See recipients of a GPG encrypted message
gpg --list-only $infile
gpg --list-only test.txt.gpg
This command outputs recipient info, including recipient key ID and details if known:
With an empty keyring (no public keys imported):
gpg: encrypted with RSA key, ID 70F28839
With a populated keyring (public key of recipient imported):
gpg: encrypted with 4096-bit RSA key, ID 70F28839, created 2018-05-29 "Josh McArthur <[email protected]>"