I installed the gstremer fluendo codecs. How do I know if I am using them or how do I specify them as an argument for vlc or mplayer?
VLC and MPlayer both use FFmpeg (+ some other) codecs to play media, but not the gstreamer ones (like fluendo-mp3). It is my understanding that this can't be changed by arguments or configuration, as it has not been implemented.
To be sure that you're using, say, fluendo mp3 decoder (flump3dec), use a command line similar to this:
Other software may provide a way to inspect which gstreamer elements it is using for media operations.
(To add to the confusion, it is also possible to use FFmpeg through gstreamer, but it is not what you want.)