The manual only gives a volume amount for the fork oil, so there's no way to tell by just looking. I have progressive springs in mine which take up more space, so the recommended volume is too much. I just set the level where I like it and use a long, thin dowel stuck down inside the fork to mark the distance from the top of the fork to the top of the oil. That way in the future when changing the fork oil I don't have to worry about volume measurments, just keep pouring in oil until it hits the mark.
Once you know the correct amount of oil to use, drain the old oil from the bottom drain plug (front wheel off the ground, of course), pour the new oil into the top of the forks, measure the level with your trusty new dowel, and never mess with volume again.