This allows you to use a laser sight to build a tool to determine if two planes are parallel. This is primarily used for aligning a photo enlarger, but can be used for any two planar surfaces.
Uses a laser sight for a pistol and a mirror. An adjustable sight allows you to adjust the beam to be exactly perpendicular to the base. When the laser reflects back into the laser aperture the two surfaces are parallel.
Buy a laser sight for a pistol that clamps to an accessory rail. Make sure you get one which features adjustment screws to aim the laser. An inexpensive sight for an air soft or pellet pistol will be fine. You can get a nice one for $17 at Walmart, or less online. You will also need a small piece of mirror, which you can get at the hardware store. you might want to have the sharp edges dressed to make it safer to handle.
Once the part is printed clamp the laser sight onto the dovetail rail with the laser pointing up. The laser aperture should be approximately centered, but it doesn't make too much difference if it's not. These directions are to align a photo enlarger, but they apply to any other two planes you wish to make parallel.
Note: avoid letting the beam hit you in the eye.
Raise the enlarger head close to the maximum height (it will be more accurate this way). Place the mirror on the negative stage. Remove the lens from the enlarger.
Place the alignment tool on the baseboard and turn the laser on. Position it so the beam hits the mirror and reflects back down. Turn the alignment tool on the base board. If the beam is perpendicular to the base the reflected dot will stay in one spot. If it circles a spot as you rotate you will need to adjust the set screws on the sight. This is the hardest part, but it's pretty easy if you do it in several iterations. First find the center position of the circle the beam makes as you rotate the tool. Turn the set screws to bring the beam to that center spot. Keep rotating, adjusting and checking until the reflected beam hardly moves at all.
At this time you should adjust you negative stage or baseboard to make the beam reflect back on itself. If you can rotate the tool and the beam always reflects back into the hole it emerged from than the two planes are parallel and the laser is perpendicular to the tool.