gpg cl-yacc-0.2.tar.gz.asc
GnuPG will complain about an unknown key, and tell you the ID. At that point, do

gpg --recv id
to download a local copy of the key.

~/Downloads$ gpg libevent-2.0.22-stable.tar.gz.asc
gpg: assuming signed data in 'libevent-2.0.22-stable.tar.gz'
gpg: Signature made Mon Jan  5 08:16:20 2015 MST using RSA key ID 8D29319A
gpg: Good signature from "Nick Mathewson <nickm@alum.mit.edu>" [unknown]
gpg:                 aka "Nick Mathewson <nickm@wangafu.net>" [unknown]
gpg:                 aka "Nick Mathewson <nickm@freehaven.net>" [unknown]
gpg:                 aka "[jpeg image of size 3369]" [unknown]
gpg: WARNING: This key is not certified with a trusted signature!
gpg:          There is no indication that the signature belongs to the owner.
Primary key fingerprint: B35B F85B F194 89D0 4E28  C33C 2119 4EBB 1657 33EA
     Subkey fingerprint: EF00 F369 1387 FCC5 8CD6  8E13 9103 97D8 8D29 319A

~/Downloads$ gpg --recv 8D29319A
gpg: requesting key 8D29319A from hkps server hkps.pool.sks-keyservers.net
gpg: key 165733EA: "Nick Mathewson <nickm@alum.mit.edu>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1