I mount a prop, and set it horizontally. Then measure from each prop tip to some point in the middle, down the back end, like the base of the fin.



A bit of trig. will then give you the thrust angle.



Something like atan ((d1 - d2)/D
)


d1, d2 are the two distances you measured.

D is the prop diameter.



This may not be exactly accurate, but sin, tan and the actual angle

(in radians) will all be close to the same value for small angles.