ORIGINAL: Geistware
Put your propeller horizontal and measure the length from the tail to the left side then from the tail to the right side. Find the difference between the two lengths and divide it by half the propeller length. THe arc sine of this number is the angle of thrust.
I've been using a similar method since '63, and it works very well indeed if you're carefull. The triangle described isn't really a "right" triangle, but for angles smaller than 10 degrees, the error is small.
For those without an engineering/scientific calculator, you can use an online one such as at math.com:
http://www.math.com/students/calcula...scientific.htm
FWIW, an arcsin is a one click calculation; enter the ratio of the side opposite the angle of interest to the hypotenuse, push the "arcsine" button and read the angle. Highschool sophomores do it all the time in trig class. Even in America.