Control surface alignment

I'm hoping the home built community can assist me with a small problem.
I own a glider that does not fly straight without applying a little
constant cross control (L stick and R rudder). If hands are removed
from the controls, it slips (or yaws?) to the left, and then the nose
gently swings around to the right. If allowed to continue, the
aircraft begins to dive to the right and rapidly builds up airspeed.

The aircraft was never damaged but may have had control rods for the
ailerons replaced putting the control surfaces out of alignment. The
maintance manual gives the proper aileron and flap deflection angles.
The angle is measured from a reference point on the bottom surface of
the wing 100mm forward of the aileron-wing junction to the trailing
edge of the aileron.

My question is how do you measure the angles of the control surfaces to
see if they are within tolerance of their deflection angles?