One at a time, press each outer string down at the first fret and near where the frets leave the body. Then check the clearance in the middle. There should be a little clearance, about the thickness of a pick, or less. If there is more clearance, the truss rod(s) need to be tightend. If not enough clearance, the rod(s) need to be loosened. Or, better yet, read "Joey's Post on setting Up Your Bass" in the "Must Reads" section.
Bill, tgo