On BF, every hill is > 8%.

But srsly, with a little digging you can probably find the grades of hills in your area. Your LBS might know some of them...or you can calculate them if you know the beginning elevation, ending elevation and distance (assuming the grade is somewhat constant). Several years ago I rode Mt. Hamilton in the Bay Area. According to several sources, the road was designed not to exceed 7% because it was going to be used to transport a telescope to the top. I think about the steepest part of that climb and then judge whether what I'm riding is steeper or less steep to arrive at a guess.