if you eye ball it , you'll get fool espacially wings with diehedrals.
close at least one eye.

makesure the wing tip have equal distance from the surface you are working on.
you can use styrafoam cups , ad spacers accrodingly.

gluing the tail section
i use a triangle to makesure the tail feather have a 90 with the fuselage.
i use a trianble that i've cut off the coner at the 90 deg if there's flairing
between the verticle stab and horizontal stab

to get the main wing or tail feather to center...i use the tape /string/ pin
method.

for the tail...
i use the triangle to draw a line useing the TE of the tail as a guide at the half
piont .
pin the string to the center of the fuselage at the main wing's TL former.

adjust the for centering accordingly to have equal distance wing tips.
the tape wrap around the string is use a marker .

once the tail section has cured. and i'm happy with the 90/90/90
fuse/horizontal/ verticle, and centering of the horizontal.

i mount the fuselage to wing and trial fit.
the tail wing's tip should also have equal distance from the working surface, if
the main wing tips have equal distance.
i sand the fuselage wing's saddle accrodly to get the tail feathers to align with
main wing or to get the tail's tip to have equal distance from the bench.

you can also use rubber foam tape as shim if you have an ARF.