I find "yum repolist" helpful in this case. Check the output and look for the repo name section. You should get an idea of what the repository is for. For example:
"MySQL 5.7 Community Server" provides MySQL packages.
"Puppet 5 Repository el 7" provides Puppet packages.
For Red Hat Enterprise Linux:
# subscription-manager repos
This will output all the repositories (along with a descriptive title) that your system is either subscribed or available to subscribe to, from Red Hat.
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