if you want to measure it very accurately you can get a SKy Quality Meter from Unihedron
www.unihedron.com
I have one and its very cool to measure sky darkness at different locations. Only problem is the old timers hate you becuase next tme they give you the ole "site x is the darkets I have ever seen etc... you now have something to measure their statement by.......
Best regards
Chris Venter