I live on a very steep hill as well. On an earlier post someone mentioned a protractor and a board to determine slope. This seemed like a pretty good idea for ballpark figures.
Unfortunately, no slope surface is perfectly flat. On my hills there are gopher holes, etcetera, all over the place and they will through off the protractor measurement for sure. Not having a clue what the incline is, I would sure like to borrow one of those tilt meters. For $50, I will probably get one.