Don't eye ball it. If you do, You have to close one eye and stand way back.
Don't trust your eyes. We all have blinde spots. I seen all those illusion pictures.

You have makesure it's correct in different directions. I usually put a mark at the center of the rear of the horizontal stab to help me.
To mount the horizontal stab.
From the TOP view makesure it's center to the fuselage. Mark the center for the front and back of the vertical stab with a pen.
It will makesure your model won't yaw, mininize drag or track straight. It is very critical becuase if you don't get it
correct, your model will yaw dfferently depending on the speed of the model.

To makesure it's 90 deg to the horizontal stab. I use a triangle or a square.
I butt the triangle to the fuselage and makesure the horizontal stab is 90 degree .
Then I mount the vertical stab and makesure it's 90 degree to the horizontal stab. I cut the conner of the triangle incase there's flairing.
If you mess up on this part it's not as bad becuase you can either sand the main wing's saddle or use wire brace to try to striaghten
the tail feathers, but i rather do it right the first time to save me a lot of work later.