Measure or count the rise over the run. You can look how far up it goes for each unit to the right or left. Or, you can find two points. They will each have an x and y coordinate (x,y). Call one point 1 and the other 2. Then:
Slope = (y(2)-y(1))/(x(2)-x(1))
For example, if two points on the line were (1,0) and (3,4) then slope would be (4-0)/(3-1) = 4/2 = 2