At least one part of that is a moral question. Moral acts, according to good moral theory, depend upon the intentions or motivations of the moral agent involved. That is not an easy question to answer, objectively speaking. Therefore, determining a “true” Catholic, because that determination depends upon a moral dimension, would be a difficult one to make because at least part of the “evidence” - the intentional world of the individual - would be largely inaccessible.
this is where things get tricky.
If a person is baptized Catholic, but then does not practice the faith…some would say they are Catholic, but lapsed, dissenting, or perhaps even ignorant, not knowing that as infants they were baptized.
Some would say these people are not true Catholics because they don’t believe or practice.
But then some of those people might actually believe, but not attend Church or receive the sacraments, but behave according to the moral teachings of the Church. According to the Church, those people are still in mortal sin because they do not attend Mass etc.
Then you have people who are perhaps married to a Catholic, or were baptized Protestant, or not baptized at all, but attend Church and receive the sacraments without anyone around them realizing they never went through the formal process of joining the Church, or were baptized (validly). They appear to be true Catholics, but their actual standing is hard to determine…they may be valid via desire or ignorance of the process.
Or someone may hate the Church and not believe any of it’s teachings but attend Mass, receive the sacraments,and follow the moral teachings etc just to keep peace in the family, or keep up appearances, or get in good with the boss at work who is a devout Catholic.
Another person may believe with their heart and soul, and truly try to follow the teachings of the Church, but because of addiction, mental illness, or some true weakness of spirit fail time and time again, be truly repentant, go to confession, only to fall again.
So, will the TRUE Catholic stand up!
Because the Church itself has some categories about desire, intent, invincible ignorance etc, there is that area in which it is hard or perhaps impossible for the average person in the pew to know whether or not the person sitting next to them (or who they see going into the porn store) is a true Catholic or not.
but I do understand, that it IS possible, that some lay Catholics might have been graced with a particular brand of spiritual discernment which allows them to “see” who is and who is not a true Catholic.
There are cases of saints and visionaries who were able to tell when a priest walked or drove past their house, or whether or not a particular item had been blessed or not. So that situation would not be unprecedented.