This is an interesting question. Note that I do not self-identify as Catholic in the religious affiliation field of the profile.
My most honest response would be that that determination is not one that a person can reasonably make for themselves. Ultimately, it is God who would best be in the position to do so precisely because “Catholic” means to be in full communion with the Body of Christ, the Church. We can work at it in various ways, agree to teachings, receive the sacraments and have a general notion that we are closer or farther from participating fully in the mystical Body, but to claim “I am Catholic” in the fullest sense of what it means to be united to Christ is about as accurate as claiming “I know the truth” or “I am good.”
It is quite presumptuous, and probably false, to make such a claim about oneself in the true sense of what it means to be “Catholic” or to be “one with Christ.”
However, I do think we can objectively identify what it means to be Catholic. Those qualities are spelled out in specific and exhaustive detail in the Gospels, Scripture and the teachings of the Church, and exemplified in the life of Christ and the saints through history.
Probably the only real answer one could give as to whether another person is actually a “Catholic” is “That is a determination best made by God. It’s above my paygrade.”
In short, the answer has three aspects:
- Self-identification as Catholic - best not even thought about - forget about it and do everything you can to be one to the best of your ability realizing full dependence upon grace.
- Objective what it means to be a Catholic - probably a good idea to read and study Scripture and the teachings of the Church, lives of the saints - do everything you can to work out your own salvation based upon everything you can ascertain from these.
- Determining whether another person is Catholic - don’t waste your time - refer them and yourself to #2