When to Go to a Finer Grit on Belt Sander?
If you change the direction the belt moves along the glass with each step it will be easier to see if you have removed each of the previous step's scratches. If you see marks at another angle than your present direction, you need to continue the current step a little while longer.
A second method that I use more often is to use a paint marker to show up the areas not ground. You need to dry the piece before applying the paint marker and let the paint dry before continuing with the next grit. Any low spots or scratches will show up as white areas or lines on the surface.
The process of drying and painting helps to provide a break in the sometimes tedious process of grinding and polishing.
These two methods also work for cold working by hand.