[EdCaffreyMS]

My answer is GOOD lighting, and taking your time to visually inspect the blade as your grinding or hand sanding. Most of the time previous grit scratches do not become evident until you've gone to the next finer grit. When this occurs your generally forced to go back to the last grit used in order to remove the scratches, and then move forward again. This get very frustrating.

When I say GOOD lighting, I also mean different types of lighting. Scratches that do not show up under florescent lights, will show up under halogen lights, likewise those that do not show up under halogen, will show up under another type of light such as sodium vapor or incandescent.

I learned this hard lesson years ago when I got to a show and was taking my knives out of the case. At that time I had only florescent lights in the shop. The show hall had sodium vapor lighting. I was so embarrassed at some of the things I had missed, that I put three knives back in the case and never offered them for sale. When I returned home, the first thing I did was install several types of lights over the finish bench. It has made a huge difference.

Its also a matter of training your eyes. The majority of students who come to my shop get frustrated because when they think their done hand finishing, I will take a sharpie marker and start marking the areas of the blade they need to work on. Most of the time the tell me that they cannot see anything, but by shifting the blade in the light, the "shadow scratches" become evident.
Its all about patience, and knowing what to look for. When someone asks me how long hand finishing takes, I tell them "Sand until you think your done.....then sand 10 minutes more."