shine Your green lasers at a diffraction grating in the same spot. A diffraction grating diffracts light at a certain angle, but the angle is different depending on the wavelength, so the distance between two dots produced by a diffraction grating at a constant distance from the grating will always be the same distance apart unless the wavelength is different.
That wasn't confusing.... Maybe if I describe what I'd do...
Ok, so, take a diffraction grating and mount it a certain distance from the wall (i.e. put it on a table and make sure it doesn't move). The larger the distance to the wall, the easier it will be to measure a difference.
Now, you need to find a way to make sure you are shining each laser at the same spot on the difraction grating, at the same angle. For this, you can use a piece of tape to mark at what angle and where you set the laser down at, or you can think of a more precise way of doing this.
From there, shine each of your lasers through the grating and measure the distance between a set of dots on the wall. Do this for all three of your green lasers, using the same two dots for measurements for each one.
One of these laser's measurements will be different if the laser is a different wavelength.
