You cannot tell a person's correct name or pronouns from their appearance.
Just because a person appears to be female, does not mean they identify with female pronouns. Just because a person's gender identity has changed, does not always mean their pronouns have also changed. A person identifying as a binary (male versus female) gender may not identify with the pronouns for that gender. If they have not told you their pronouns, ask them.
"Excuse me, I would like to respect your gender identity. May I ask which pronouns you use?"
Until you know a person's correct pronouns, use they/them/theirs to avoid making an incorrect assumption and possibly offending them or triggering feelings of gender dysphoria.
Introduce Your Pronouns with Your Name
Whenever you introduce your name, also introduce your pronouns. This lets everyone you meet immediately identify you as a trans ally and know that you are someone with whom they can feel safe expressing their authentic selves. It also encourages people to provide their own names and pronouns without your needing to ask for them.
"Hi, my name is Arien Reed and my pronouns are he, him, his. And you?"
You can also insert your pronouns just after your name in your professional email signature.
Arien Reed, MFA
My Pronouns: he, him, his
Artist, Writer, Musician
You can also put them right after your name on a name tag.
Respect & Practice
Now that you have identified a person's correct name and pronouns, it is important to do your best to always use them. When speaking aloud, make sure you are using a respectful tone and body language. The portrayal of a negative attitude, even if unintentionally done, can cause the person to feel mocked or bullied, or to feel as though they do not belong in the current environment.
Apologize & Correct Yourself
We are all human, and humans sometimes make mistakes. When you accidentally use a wrong name or pronoun, always apologize as soon as possible (even if you don't realize your mistake until months or even years later). Keep the apology a quick one and do not go on about how bad you feel about the mistake; a long apology can make the trans person feel awkward and it puts them in a situation to comfort you, which is inappropriate and not their obligation. Keep the apology and related bodily language respectful, so it is not offensive.
HELLO. My name is...
he, him, his
"Alana—Oops! Sorry. Arien."
"She—I mean, he doesn't like pizza."
"She—I mean, he. Whatever." (rolls eyes, makes a dismissive hand gesture, etc).
A respectful apology shows you did not mean to disrespect their identity. And remember, there is no expiration date on apologies! It can mean a lot to someone that you thought about them and valued their identity enough to apologize for your mistake even months or years after you made it. It's never too late to show you care and that you never meant to hurt anyone's feelings.
If you make the mistake often, promise to continue to make the effort to correct yourself. Then keep your promise (following these tips as needed).
"Hey there, I'm sorry I referred to you with the wrong name and pronouns yesterday. I will keep trying to do better, I promise."
Immediately correcting yourself, and promising to do better in the future, shows the trans person matters to you enough that you will continue to make the effort on their behalf, even though you find it difficult or may not understand their identity. But make sure you live up to your promise, and practice, practice, practice.
It is usually both appropriate and welcome to support the trans person in your life by gently correcting others on their behalf whenever they are misgendered or dead-named. This can also be done by informing the person making the mistake, or if you believe they are already informed, then by immediately saying the correct pronoun or name after the wrong one is spoken.
"Arien's correct pronouns are 'he/him.'"
It is usually best to address such pronoun and name microaggressions whenever they occur, even if it is someone of high authority doing it. This lets the trans person know that you are their ally and that your office, classroom, or personal space is a safe place.
Ask Before Getting Involved
When a person or group of persons displays an ongoing habit of misgendering or dead-naming a trans colleague, student, friend, family member, etc, before privately handling the situation with the offender(s) themself(lves), it may be appropriate to approach the trans person and ask, "I noticed that you were being referred to by the wrong name and pronouns recently, and I understand that can be hurtful and triggering. Would it be okay with you if I took them aside and reminded them of your correct name and pronouns?" Do not proceed if the trans person seems uncomfortable with this suggestion or explicitly declines it. Regardless, they will likely still appreciate your helpful intentions.
Pronouns to Avoid
"It" is not an appropriate pronoun to use when referring to any human being and "he-she" or "he/she" are offensive slurs commonly used against trans individuals.