It cannot be stressed enough that everyone has the right to privacy. A person's transgender status should always be treated with the same high level of confidentiality as any other sensitive personal information.
Some people may be happy to have certain people know they are transgender, but not others, so even if they appear open about their transgender status & leave it up to the transgender person to decide who they wish to tell.
Revealing someone is transgender ("outing" them) not only violates their right to privacy, it also places them at risk of discrimination and harassment. It can sometimes even place them at risk of physical or sexual assault.
Inappropriate questioning is anything that is of a higher level of intimacy than questions you would ask a person who is not transgender. For example, it would be unthinkable to ask a man you didn’t know very well about the size and shape of his penis, or ask a woman you only knew a little whether or not she wore a wig or a padded bra, so it is also completely inappropriate to quiz transgender people in this way.
Don’t let any natural curiosity about transgender people override your usual politeness and sensitivity. Each transgender person is not obliged to be a public spokesperson for the whole transgender community, so don’t expect them to want to talk about the subject anywhere at anytime.
If you have questions you want to ask, which you think are appropriate, phrase them politely and carefully choose a suitable time.
If the person says they would rather not discuss something then don’t pressurise them or else they will just end up unwilling to talk to you about anything.
If you know someone who wishes to change the pronoun they are referred to by (she to he, or vice versa) try to understand it is important to them that you respect this, even if you initially find it difficult to consider them that gender. Deliberately using the opposite pronoun to the one a person prefers will be experienced as offensive, judgemental and hurtful.
If you are speaking briefly with someone and you are unsure whether the person would wish to be addressed as he and sir or she and ma’am, then it is usually best just to avoid saying either than to risk insulting them by guessing wrong.
When the interaction is long enough, ask the person their name to try to determine which pronoun to use. If it is not clear whether their name is a male or female name then it is acceptable to politely ask: ‘excuse me, but which pronoun do you prefer?’ or ‘excuse me, but how do you prefer to be addressed?’
If you accidentally use the wrong pronoun, just apologise once and continue with the conversation. You don’t need to apologise profusely or try to explain why it happened - this often only causes more awkwardness.
When referring to a transgender person in their absence, you should still use their preferred pronoun, not only out of respect for their identity, but also because it will help to prevent confusion, uncertainty and embarrassment for everyone. Occasionally the situation can be a little more complex: A transgender person may prefer one pronoun and name in some situations, and a different name and pronoun in others. In such cases it is still very important to try and get it right. If you are ever unsure, ask them again & don’t just guess. It is also important not to link the names together as that could lead to them being outed. When writing about a transgender person, do not belittle their identity by putting their preferred name or pronoun in quotes or italics as this suggests their identity is less valid then everyone else’s.