How can I tell if a third-party repository is reputable?

Among various other points, I intend to install ffmpeg on a CentOS web server of mine (simply an individual web server - absolutely nothing also unique).

I saw that set of installment guidelines suggest to add a DAG repository:

[dag]
name=Dag RPM Repository for Red Hat Enterprise Linux
baseurl=http://apt.sw.be/redhat/el$releasever/en/$basearch/dag
gpgcheck=1
enabled=1

The name appears acquainted yet I'm not exactly sure if I've utilized this set in the past. Just how can I inform if it is trustworthy / secure / trusted?