In order to find the angle you need to use a little trigonometry. Using a side wall that runs up to the ceiling pick a point on the wall. From that point using a level measure straight up to the ceiling. Then measure straight and level to the right to a point where the wall meets the ceiling again. Now measure the distance between the two points where you hit the ceiling. You should now have three measurements for a triangle. Go here for an online calculator to help you once you have the measurements.
http://www.webmath.com/rtri.html