There is a difference between judging the person, and judging the behavior. We should not judge persons, but we should judge acts (behavior). If a person robs a bank, we say that bank robbery is wrong. But we don’t say, that person is going to Hell.
If a person tells a lie, on one occasion or on a few occasions, we can condemn the lie, but not the person. We also cannot call such a person a liar, because he does not lie frequently or habitually. On the other hand, if a person is known to lie frequently, on a continuing basis, it is a fact that the person is a liar. Similarly, if a person steals repeatedly, habitually, the person is a thief.
These are objective statements about objective sins. If to some extent the behavior reflects on the person, that is not of our judgment, but of the objective facts about that person’s choices. So, it is possible to make some statements about the mind and heart of a person who is known to be committing objectively sinful acts, if they reveal their mind and heart by their own words and actions.
For example, if someone asserts a heresy, we can say that the person has asserted material heresy; objectively, the claim is heretical. However, if a person asserts a heresy, and asserts that he knows that the Church definitely and irreformably teaches the doctrine being denied, and asserts that he is nevertheless rejecting that teaching, his sin is formal heresy. We are not judging his mind and heart, but he is revealing his mind and heart.
Can we say that a person is a bad Catholic? Only if the person had revealed, by their objective words and actions, that the person is committing grave sin on a continuing basis, without repentance, or that the person is committing formal heresy on a continuing basis, without repentance. In other words, we could only make such an assertion about the person, if it is known to be objective truth, not a subjective judgment.
Some examples: A person is objectively not a Catholic. He does not practice the Catholic faith at all, and he does not believe the Catholic faith at all. If he asserts that he is a Catholic, we can say that he is not, because it is objectively true. But we cannot say that he is or is not in a state of grace, or whether or not he will go to Heaven.
Suppose a person is a baptized confirmed Catholic, who attends Mass. But the person is known to be divorced without an annulment, and to be living with an illicit lover, and (as a politician) to be a supporter of abortion and same-sex marriage. We can say that the person is a bad Catholic because the person’s objective words and actions on a continuing basis are clearly incompatible with the Catholic Faith.
Can we judge that a person is or is not in a state of grace? Rarely. For example, if (at a time before her death) we considered the life of Mother Teresa of Calcutta, we would conclude from her well-known words and deeds that she is a faithful Catholic who is very holy. Similarly, if we considered the life of Saddam Hussein, we would conclude that he is a wicked person who is not in a state of grace. This type of judgment is not an article of faith and is not infallible, but is relatively certain as far as human knowledge is concerned.