You're starting out right...

1 Make sure the stab is centered
2 use a tape measure to check the distance from the wing tip to the stab tip
3 Eyeball the stab against the wing (as the others have said) Close one eye and align your eye with the center of the fuse. Slowly raise or lower your eye to get the top of the stab to line up with the bottom of the wing (or vice versa).

If the stab "Meets one side of the wing while there's still a gap on the other side, the stab is tilted. Move it so that it aligns with the wing, and using slivers of balsa, shim it in place. Add a drop or two of Medium CA to a few critical areas (with accellerator) to hold everything in place while the epoxy cures.

Once the epoxy has sat at least a few hours (I'm talking 30 minute epoxy, don't use anything shorter for this) You can fill any gaps that may have formed from the shims.