How do you distinguish the fine line between if someone rejects the Catholic faith vs. someone just not being aware of the fullness of the Catholic faith?

The first thing I wonder on reading your question is why you feel the need to make such a distinction? Jesus commands, “Stop Judging that you may not be judged” (Mt 7:1). St. Ignatius counsels that the first question one should ask if one feels the need to correct another is if one is, in truth and love, the one to make the correction. Maybe the correction will be better received and appropriated if the appropriate person suggests the correction.

On another level, the answer to your question is rather obvious: ask the person. Only a person in the sanctity of their own conscience can decide if he or she is rejecting the Catholic faith. Only the person can tell you if they are rejecting the faith. And, even if they do so for a time, they are always welcomed home if they want to return.

After more than 30 years as a Jesuit, I can tell you our faith is very full and rich. Everyday I learn more and more about Catholicism. I would bet that what looks like a rejection of the faith to one person is very often unawareness on the part of the person perceived to be rejecting the faith. Even more, our faith tradition has many styles, literally differing rites. There are many ways to be Catholic (e.g., Franciscan, Ignatian, etc.). Just because someone’s way of being a Catholic differs from your own does not mean that person is rejecting the faith.