You need to shoot from an angle - if all the trees/signs are in front of you then they will all appear vertical in the image regardless of the slope.
This is actually a well known illusion that gives rise to "gravity hill", a road which slopes downhill but looking head on in the absence of any visual cues the brain interprets it as flat/sloping up, which causes objects to appear to roll uphill!
Photo by fluxn
If you shoot side-on or from an angle there would be a clear difference in the line of the trees and road which would give the viewer the information that the road is sloping.
Here's an extreme example:
The slope is clearly evident when you look at the trees. Note that photo was taken by a friend with my camera, as I'm in it!