How do you know if peripheral vision loss is minor. Field test says normal but there are some gray shaded areas around part of the periphery.

Depends. . This depends on many factors. The size of the visual field test, how wide an area was it measuring. It also depends on how stable the change is- is it new or has it been like this for many years. Also how severe the changes are- the sensitivity is expressed in "decibels". Small changes are ok unless they exceed what you would expect for that sight tested based on past tests. Talk to your md.
Minor in what way? Some individuals may notice mild field loss, but most do not and perhaps for this reason we refer to it as mild. The visual field analyzer computer does a rapid statistical analysis on the study and rates the responses against normal non glaucomatous patients. If you look at the normal distribution of vision, many people can have mild field loss that is not considered clinically significant.