When you are checking for parallel, you must stand back several feet. I like about 15 feet to get a good perspective. I find it I just hold it at arm's length, I can't get a good alignment.

Next, once you have backed off, move your head up or down until you see a gap betwen the stab and wing. Then move the other way until one side or the other appears to touch the wing. if one side touches first, the stab isn't parallel.

I also like to use 30-minute epoxy. I line it up to start with and level the gluing area. Then I remove the stab and coat it with glue and re-install. Now I go back and re-check the alignment. The slower curing epoxy lets me make last second changes.