Which of the 4 types: Blazed, Unblazed, Scribed or Unscribed?
If blazed or scribed:
Just look at each surface in a well lit environment...The shiniest "rainbow" like reflection will be the ruled side.
If unblazed or unscribed:
It doesn't matter, either way the light will be incident on the gratings and transmit the diffraction pattern.
Hope that was clear :)