You may need to know an angle for use with the "Draw/Sweep" (W) or rotate (R) functions. There are at least three ways to do this. 

1. The "Dimension/Info/Angle between two lines" function is a quick way to measure an angle if you have two existing lines, even though it is buried three levels deep in the menu structure. The lines must be co planar and nonparallel, but the line segments do not have to cross. They can lie in any plane, so this function works well in 3D mode. The result is displayed in a dialog box that is easy to read. 

2. If you actually have an arc in the drawing you can select it and open the Info Box (CTRL I) to find the "Span Angle." It must be a true arc, and not an arc drawn as line segments. 

3. You can measure an angle with the "Dimension Angle" function. Set a point at the center of the arc and points at the ends, then a point to set the distance of the text from the center.