Personally, I’m very much out and visible as trans, so chances are if the person has done any looking, they’ve already found out. But that’s me. There’s nothing wrong with not being public about being trans – it’s a personal decision.
So when? Whenever and if ever you deem it safe and appropriate. You don’t have to disclose. And anyone who tells you that you must (especially in cases of dating) is perpetuating the notion that cisgender is preferred and somehow more valid and/or valuable. For what other things is there an obligation to disclose specific aspects of your history? I’m hard pressed to find any.
That said, sharing your history is generally a sign of trust and vulnerability, so by disclosing, you may be able to foster an environment of mutual trust and respect. But there’s always the few assholes out there who mess it up for everyone else. So be safe, and whatever you decide, it is your decision and yours alone.
Once a transgender person has fully transitioned surgically, do they have an obligation or duty to forewarn potential partners?
Let’s be clear: a transgender person is under no obligation to disclose that they are transgender, regardless of surgical status or any other aspect of their transition. That is privileged medical information and one cannot be required to disclose it under any circumstances.
Let’s examine the motivation for this question: why would a person be required to disclose private medical information? In the case of communicable diseases / STIs, there is a potential health risk for a sexual partner. In something like paralysis, the person might need an accommodation in order to participate in an activity or society in general.
Being transgender is not communicable – you can’t catch it by contact with a transgender person. It represents zero threat to anyone else. There is no reason to oblige a transgender person to disclose it.
I’ve heard some make the argument that because a transgender person is unlikely to be able to be fertile, they owe it to their partner to disclose. But this burden is not placed on other people who might face issues with fertility: people suffering endometriosis, low hormone levels, or even those who have suffered trauma to their gonads. It is not acceptable to single out transgender people to be uniquely obliged to disclose their possible reproductive infertility.
No, when you get to the root of the issue, the reason most proponents of obligatory disclosure have this position because of some fear or hatred of transgender people. They are afraid of having their sexual orientation questioned because of being attracted to a transgender person, or have some other basic (often irrational) fear.
All that said, I would hope that the transgender person would be in a secure and loving place in their relationship where they would feel safe and supported in sharing this information out of a sense of wanting to share intimate parts of themselves with the other person, and not out of any obligation.