The things i always look for are adequate ventilation, good lighting, and heat. I don't like seeing by breath in an indoor range.
Make sure the bullet trap is solidly built and designed. I haven't started looking for a range in the Dover area yet because my gun safe is still in VT with my dad.
I know that there is a good one in Dunbarton, called Pioneer Sportsman. When i saw their range, i didn't see too many bullet holes above head level, which is a good indication that they don't have too many nincompoops shooting.