I found this cool command called glxgears that you can run under X.

$ glxgears
10524 frames in 5.0 seconds = 2104.650 FPS
10003 frames in 5.0 seconds = 2000.501 FPS
8417 frames in 5.0 seconds = 1683.340 FPS    * Moved the window from left to right
10538 frames in 5.0 seconds = 2107.443 FPS
5639 frames in 5.0 seconds = 1120.850 FPS   * Moved the window around constantly
X connection to :0.0 broken (explicit kill or server shutdown).

$ glxgears --help
Warrning: unknown parameter: --help

Note the spelling error in “Warrning”.