How can you tell if GStreamer plugins are installed correctly

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http://unix.stackexchange.com – I've installed GStreamer plugins for Fedora 14 using yum. However, when running a qt-phonon application with the GStreamer backend, I get the error "Warning: You do not seem to have the base GStreamer plugins installed. All audio and video support has been disabled" Is there a good baseline test to determine if this is a Phonon problem? Does anybody know where yum installs the GStreamer plugins and how GStreamer resolves plugins? (HowTos)