As has been mentioned before, the hicksons make for a great set of objects to use to gauge the current sky conditions. In the late summer I often use the 'fleas' of 7331 for this sort of test but hicksons are all over the sky so picking 6-8 with a wide range of magnitudes offers us a sort of sky condition 'scale' for a given instrument to better judge transparency. Seeing is easier to put a number on than transparency from my experience but perhaps I lack the 'tricks' of the deep sky experts in putting a number on transparency alone.