♪♫♪♫Up the Hill I go♪♫♪♫

This may be a silly question but im asking anyway. I have a steep hill by my house I would love to start climbing. I can get up to a 6% on my treadmill (not for long) but I wanted to change up things a little. My question is How can I tell what the 'Incline' is of the Hill? I dont have one of those HRM monitors or watches.