Answering my own post!
How do I know that a package is a EPEL package?
All EPEL packages are signed with the EPEL gpg-key. The public key with id 217521F6 is part of the epel-release package for epel 5; The public key with id 0608B895 is part of the epel-release package for epel 6; yum will ask you to import it the first time you install a EPEL package. See also: https://fedoraproject.org/keys
Something I failed to mention in my problem description was that I was installing my first EPEL package (WINE) on CentOS 6.4. The fact that it is CentOS 6.4 is irrelevant, and only included for passing interest. The fact that it was the first time I tried to install an EPEL package is extremely relevant to the error encountered.
Literally, I installed CentOS 6.4 and the first thing I did was add the EPEL repository to yum, followed by "yum install wine". This took me HOURS to accomplish, because I had to learn about packages, then repositories, and finally yum.
To add the EPEL repository to CentOS 6.4,
sudo rpm -Uvh http://mirrors.kernel.org/fedora-epe...6-8.noarch.rpm
if you are running 64-bit version of CentOS 6.x.
The same tree will bifurcate after [/6/] into a path for x386 as well to the same named rpm.
One site said it didn't matter what architecture your machine was (32 or 64 bit), you could use either rpm, but then why would they create two separate filesystems for each architecture? Didn't make any sense to me, so I opted to go with a more conservative approach that honored the filesystem difference.