If you have Google Earth (even the free version), you can pretty easily calculate slope angles. A GIS program is a bit more accurate, because they use more detailed images and you very accurately lay down the run coodinates. Here's how you do it in Google Earth:
1) Find you desired run. In this case Baldy Chutes @ Alta.
2) Tilt/Zoom in until you can get a good view of the run. You can look sideways at the run to see where the steepest section is.
3) Use the Measure Tool, set to Line and click a begining coord and ending coord. Set the length measurement to feet. Make sure you grab at least 700'.
4) Hover the mouse over the top coord. Look down on the bottom of the frame, and it gives you an elevation, in this 10,707'. Do the same on the bottom coord, in this case 10,191'. Subtract to get rise, in this case 516'.
5) Now divide by the measured length, in this case 700.83'.
6) Take the sin inverse to get the slope angle, in this case 47.4 degrees, rounded off to the nearest tenth of degree.