I usually use a 1'' 23ga in the glutes but that is only when I am leaned out. For the most in the offseason I use a 22ga 1.5". If you are fairly lean you shouldn't have any problems just make sure you stick it in all the way.
It all depends on how lean you are, but I would never stick myself with and 18 gauge harpoon no matter what. If your ass isn't fat 1 inch should work. Just bury it.
I think you can tell if you did it right by how much it bleeds and whether or not any oil comes leaking back out. I know when I go deep on my quads with either a 1" or 1.5" there is hardly any bleeding and nothing leaks out, but if I go to shallow I will see some of the oil start to seep back out. If you can do it without any oil leaking out then I would think you were okay to use a 1".
i am no fat ass and i stick to the 1.5 inch just to make sure it gets deep into the muscle. the length doesn't matter as far as pain goes...its all the same once it breaks the skin!