Sometimes you need to check whether you Linux 3D acceleration is really working (besides the glxinfo output). This can be quickly done by the glxgears tool. However, the FPS are often limited to the displays vertical refresh rate (i.e. 60 fps). So the tool becomes more or less useless since even a software render can produce 60FPS glxgears easily on modern CPUs.
I found it rather hard to get a quick and easy solution for this, I answer my own question. Hopefully it saves your time.
The vblank_mode environment variable does the trick. You should then get several hundreds FPS on modern hardware. And you are now able to compare the results with others.
$> vblank_mode=0 glxgears