kohen priest - How exactly is a Zona labeled a Zona

Does the fact that a Zona (who becomes Forbidden to a Kohen) enters that status depend on the act that happens, or does the nature of how it happens, and what evidence we have about it, matter? It would seem that if the act had to be witnessed, then the problem would be the public nature of the sin (with two reliable Aidim) rather than the actual act itself.

If it is just dependent on the action, is it an issue between her and Hashem? So if she does not come forward or there are no witnesses would there be issues with her marrying a Kohen? Do we say that the act alone is the issue?

-or-

Do the circumstances need to be as such that only if she were so brazen THAT two kosher witnesses would be available at the time. (probably due to frequency and lack of caution and openness in such acts)

Even if that isn't the whole problem, are special consideration in a case when a woman had relations with a gentile and did not know they were wrong as opposed to someone who was raised frum and knew what she was not supposed to do?

And is there any distinction between the Zona status of a woman who had a forbidden relationship with a gentile vs. a woman who had an incestuous or adulterous relationship?

Answer

The Rambam (Isurei Biah 18:6 (English)) rules regarding when a woman becomes forbidden to a Kohein as a Zonah:

כל הנבעלת לאדם שעושה אותה זונה--בין באונס בין ברצון, בין בזדון בין בשגגה, בין כדרכה בין שלא כדרכה--משהערה^* בה, נפסלה משום זונה: ובלבד שתהיה בת שלוש שנים ויום אחד, ויהיה הבועל בן תשע שנים ויום אחד ומעלה.‏

Whenever a woman engages in relations that cause her to be deemed a zonah, she becomes disqualified as soon as the man's organ enters her^* whether she engages in relations against her will or willingly, whether in conscious violation or inadvertently, whether through vaginal or anal intercourse. [This applies] provided she is at least three years old and the man with whom she engages in relations is nine years old or more. (translation from chabad.org)

The Shulchan Aruch uses almost the exact same wording in Even haEzer 6:9.

See this answer regarding which partners render a woman a Zonah.

Please ensure that an actual qualified Rabbi is consulted and given all relevant details when seeking a ruling for a real life question.

^{* See Issurei Biah 1:10 et seq.}