I built this clinometer for my road bike, to go on the top tube. It's
basically an aquarium tube filled with sewing machine oil and a ball
bearing. It has markings for plus and minus 10% road grades.
http://www.keystandard.com/clinometer.jpg
I think this will work better than a bubble. I'll need to wait for the
snow to melt before I road test it.
But why stop there on instrumentation? Did you ever think that if you
had an anemometer (wind speed thingy) attached to the front of your bike
then you could subtract its speed reading from your actual road speed to
learn if you had a headwind? Has anyone here ever tried this?