Like the title says :
$> gst-inspect videomixer
Factory Details:
  Long name:    Video mixer
  Class:        Filter/Editor/Video
  Description:  Mix multiple video streams
  Author(s):    Wim Taymans <wim@fluendo.com>
  Rank:         primary (256)

Plugin Details:
  Name:                 videomixer
  Description:          Video mixer
  Filename:             /media/extra/usr/lib/gstreamer-0.10//libgstvideomixer.so
  Version:              0.10.6.1
  License:              LGPL
  Source module:        gst-plugins-good
  Binary package:       GStreamer Good Plug-ins CVS/prerelease
  Origin URL:           Unknown package origin

GObject
 +----GstObject
       +----GstElement
             +----GstVideoMixer

Pad Templates:
  SRC template: 'src'
    Availability: Always
    Capabilities:
      video/x-raw-yuv
                 format: AYUV
                  width: [ 1, 2147483647 ]
                 height: [ 1, 2147483647 ]
              framerate: [ 0/1, 2147483647/1 ]

  SINK template: 'sink_%d'
    Availability: On request
      Has request_new_pad() function: gst_videomixer_request_new_pad
    Capabilities:
      video/x-raw-yuv
                 format: AYUV
                  width: [ 1, 2147483647 ]
                 height: [ 1, 2147483647 ]
              framerate: [ 0/1, 2147483647/1 ]


Element Flags:
  no flags set

Element Implementation:
  Has change_state() function: gst_videomixer_change_state
  Has custom save_thyself() function: gst_element_save_thyself
  Has custom restore_thyself() function: gst_element_restore_thyself

Element has no clocking capabilities.
Element has no indexing capabilities.

Pads:
  SRC: 'src'
    Implementation:
    Pad Template: 'src'

Element Properties:
  name                : The name of the object
                        flags: readable, writable
                        String. Default: null Current: "videomixer0"
  background          : Background type
                        flags: readable, writable
                        Enum "GstVideoMixerBackground" Current: 0, "checker"
                           (0): checker          - Checker pattern
                           (1): black            - Black
                           (2): white            - White
Segmentation fault (core dumped)