This is pretty simple: $ xfce4-power-manager -V -- no output -- $ xfce4-power-manager --no-daemon -V -- shows version information -- seems that the first version should work aswell.
This was fixed in xfce4-power-manager 1.3.2: http://git.xfce.org/xfce/xfce4-power-manager/commit/?id=64cd25f7d31ecbb82f53bceac0e5596338b431e9