If you are interested in an accurate method of measuring kerf width, a cell phone camera and any photo editing software that has a pixel ruler can help. Since kerf depends on laser power and material, this can be useful for measuring any material on any cut, engraving or etch.
Have the laser cut a test line at the power you want to measure on the material you want to use. Take a photo of the line with a ruler in the field of view. Take the photo as close as possible to the material ( a cell phone is about 2" focal limit)
Within your photo editing software, take xy coordinates of the ruler and determine the number of pixels/mm. since the ruler or lines are rarely square to the photo, use d= sqrt(x^2+y^2) I take the measurement across a number of mm in the photo to increase accuracy. now that the photo is calibrated, any line or etch on the photo can be measured.
This method should be able to provide micron precision (.001mm)