On newer versions of yum, this information is stored in the "yumdb" when the package is installed. This is the only 100% accurate way to get the information, and you can use:
yumdb search from_repo repoid
(or repoquery and grep -- don't grep yum output). However the command "find-repos-of-install" was part of yum-utils for a while which did the best guess without that information:
http://james.fedorapeople.org/yum/commands/find-repos-of-install.py
As floyd said, a lot of repos. include a unique "dist" tag in their release, and you can look for that ... however from what you said, I guess that isn't the case for you?