gpg --list-only $infile

e.g.

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]>"