I'd like to know about this, too.
Tell me, would the seperation distance, and parallelism not be taken care of by the gantry itself? If not, a rod and a set of feeler gauges will work to space the rails precisely
Is parallelism to the ground actually relevant? Parallelism to the table is more important and can be measured between the gantry and the table with a micrometer.
This leaves just a possible twist of the rails out of one plane as the problem. So would the following procedure work?
Mount your laser at one extreme end of your gantry, mounted on the cross slide. Point the laser roughly parallel to the long X rails, shining on a target some distance away, the farther, the better, I suppose. Run the gantry back and forth full travel of the X rails and adjust the aim of your laser so that it never moves off of a spot on the target. When this is achieved, then you should move the cross slide across to the opposite side of the gantry. Without touching the laser alignment whatsoever, set up a second target and examine the position of the spot on it. If it moves up or down as you move the gantry full length down the rails, then the rail on this side has some twist out of the plane of the other rail.
You tell me if this is easy to do