Answer You build a calibration jig that wraps around the blade and has a flat surface parallel to the chord on which you place a protractor. With the blade locked horizontal you measure the blade angles relative to the plane of rotation as a function of radius. That gives you the variation of pitch from root to tip (twist) and the value at 3/4 radius is often chosen as the reference pitch angle.
Alternatively, you find the leading edge and trailing edge x,y,z coordinates and compute the twist geometrically. That information is often developed as part of the propeller design.
Aeronautics, Aerodynamics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.
38 years as research engineer at NASA
Publications AIAA, NASA
Education/Credentials B.S. and M.S. Aeronautical Engineering - U. of Washington,
Graduate work Standford U.
Awards and Honors AIAA Associate Fellow (American Institute of Aeronautics and Astronautics)