Originally Posted by
excitedbox
Late to the party, but a laser pointer works. Mount the laser to your rail using the straight edge or slide carriage. Pick a wall as far away as possible in line with your rail and make a dot on it. Point the laser at the dot. Now slide the laser down the rail and as long as it is pointing at the dot, the rail is level. The further the distance to the wall is, the more accurate you can be. Just like a rifle, a small deviation of the barrel makes a large difference at the target.
You can use mirrors to bounce the beam back and forth to increase your distance. If you want to level 2 rails, then you need to lay a straight edge across them and level both ends of the straight edge, on both ends of the rail.