What are the appropriate plugins and how do I find out if I have them? I have an Intel card and normally use mplayer-vaapi[1] to get hardware acceleration for decoding.
OT: Check out http://mpv.io/. It's a forked mplayer2 with native support for libva:
mpv --hwdec=vaapi --vo=vaapi
[1] http://gstreamer.freedesktop.org/documentation/plugins.htmlhttps://bugzilla.mozilla.org/show_bug.cgi?id=806917
Gstreamer 1.0 is a work in progress.