One of the best ways to evaluate observing conditions is to determine the telescopic limiting magnitude. Only when the seeing and transparency are at their best, will your telescope push the performance envelope. Open star clusters are excellent tools in this endeavor. So long as you have a good chart of the cluster with accurate visual magnitudes that reach well beyond your telescope's threshold, you can perform this test.

Roger Clark's Visual Astronomy of the Deep Sky is a valuable resource for this kind of work. "Appendix B" in Clark details several open star clusters which can be used for a telescopic limiting magnitude check. NGC 6823 is among these. This cluster is found in Vulpecula and features stars ranging in brightness from magnitude 9.5 to 15.9, perfect for my 10-inch Newtonian.

The sketch at left, oriented with north to the right, records the view at 258X in my 10-inch, f/4.5 Newtonian. Forty-seven stars are drawn, the faintest confirmed at magnitude 14.5. This is about one full magnitude brighter than the theoretical limit for a 10-inch aperture. Conditions must have been less than optimal that night, as I've gone to magnitude 15.7 while observing in the vicinity of M57.