Discus Winglets

CS is provided with tip ribs installed for winglet mounting so this
should be quite accurate.Rib angles out 2mm over about 250mm chord at
cut point. This is shown in TN 360-13 describling installation.
As to angle of the flying surface measurement,place winglet root rib on
suitable surface and measure distance of leading edge and trailing edge
from ref surface at known spanwise point. With this information, you
can determine if they are the same and what the angle is.
Early winglets produce by Peter Masak has the angle out(toe) at a much
larger degree than more recent designs to try to avoid stalling at high
angles of attack. This results in poorer performance at high speeds.
More recent designs use improved airfoils and reduced toe to load the
winglet more helping low speed while improving the high speed cross
over point.
Maughmer designed winglets provided by M&H are an example of this
approach and work very well.
Hope this is of some help.
UH