As you pass from one grit to another, it is often difficult to tell if you have really removed the grinding marks from the previous grit.
By using a paint marker, you can mark at random over the ground surface. Let it dry before you began the next grit.
As you grind with the finer grit you will be able to check that you have removed the glass down to the depth of the deepest scratch by the absence of the paint. Of course drying the glass and making a visual check of the surface will provide insurance. If you are satisfied with this stage, paint again before changing to a finer grit. This allows the paint to dry before you begin grinding again.