"To begin with, do you know your partner's name? And by that I don't mean their preferred name. I mean their NAME."
Let's begin, shall we?
Names
You may or may not know your partner's gender status before you begin dating them. However, if they do happen to be trans* or queer, or sympathizing with the terms, and you are cisgender, this article may be something for you to consider.
To begin with, do you know your partner's name? And by that I don't mean their preferred name. I mean their NAME. It is not just something they prefer; it is not something you can conveniently slip on and off as you wish. It is their name, or names, if they identify as having multiple ones. The same goes for their pronouns.
Also, be sure to ask which name to use when. It is possible that your partner is still using a different name around different folks. But do not pry--- do not ask, "What was your old name?" or "What is your real name?" Just accept that their name is the one that they tell you.
Legitimacy
If one day they change their name online on, say Facebook, depending on where you both are in the relationship, it can help to politely question it. For example, you could ask, "Should I start referring to you as this at all times, or around certain people?" I am ambivalent toward asking, "Did you change your name so coworkers or family could not find you?" Why they changed their name is their own business, and the same trap that trans* people fall in when they try to change their name legally--- often, they are asked, "Are you only changing your name to escape this entity?" (For example, debt collectors.)
Do not rely on legality to determine the legitimacy of your partner's name. Changing your name can be a difficult process--- not everyone has the privilege of being able to go to a court date or pay fees. Also, some may not feel safe to announce their transition, via name change, to employers.
Defining Roles
Because this article is about dating, it also helps to define roles in the relationship, and if you want to define roles at all. Either way, you should ask your partner which title you should call them, and if the title should ever change, according to circumstance. Maybe it is okay to call them "daddy" in bed, but would like to be your "girlfriend" around friends, and your "partner" around strangers, etc. While this is good to do in all relationships, this can be especially crucial in trans* and queer relationships, as some may be living in "stealth" (a concept similar to "in the closet"). That is, they may pass as whichever gender they intend to and do not wish for that gender to be challenged; some may question the legitimacy of their gender once they find out that person's past may not reflect the current gender they are perceiving today.
Community Exposure
If you do not know much about trans* or queer communities, read up on trans* articles and books. Watch trans* movies, and become cultured, but critical. Do not assume that everything they are presenting is some essential truth, or that trans* or queer people could ever be summarized down by a single experience, but try to understand that there are shreds of "commonality" throughout that media. Watch YouTube videos, but remember also that many of those people are privileged enough to have access to recording means and the internet. Remember that many who are trans* may not have the privilege to "transition."
Respect
Some may not have the means to get the hormones, name changes, surgeries, etc. that you may associate with "transitioning." For that reason, do not say your partner is not trans* or queer simply because they have not done any of those things. Similarly, do not call them trans* or queer just because they fit stereotypes you know. Some may not identify as being a trans* man or trans* woman or trans* person, for example, but may identify as "a woman who happens to be trans*," or "just a woman." Respect that.
Ask your partner if it is okay for you to correct others when they misgender your partner, and how. Again, some may be living in stealth. Tell your partner you are asking, for this reason, not because you are questioning the legitimacy of their gender.
Don't Just Respect Your Partner; Respect Everyone
To set a good example, do not assume others' genders around your partner, either. For example, if you two are together, do not say anything like, "That guy, our waiter over there..." Avoid gendered terms at all. If you do not assume around your partner, they may see that you respect all genders, regardless of relationship status--- you are not merely putting on a show for them. At the same time, though, remember that some may take offense toward being referred to neutrally. They may say, "What? You couldn't tell what my gender was just by looking at me?" So that is something to consider.
Consent and Negotiation
Also be sure to ask what if it is okay for you to touch, and how. Always use consent, and if consensual non-consent is involved... well, that's for another article! Just be sure to pre-negotiate, and remember that some may fall into "space" (something similar to dom/domme space, or sub space). It is better to ask ahead of doing physical acts together what is okay/not okay to do.
Ask which clothing articles are okay to remove ahead of time--- ask this about your own clothes and your partner's clothes. Some trans* people may feel dysphoric when seeing others' bodies. It may cause them to think about the things they hate about themselves, or things they wish they had. Ask what your partner calls their parts, and before that, ask if they are even okay talking about them. Some may feel too dysphoric to do so, but some may also take pride in the self-determined labeling of their body.
Autonomy and Labels
Do not label your partner simply to support the reasoning in your sexual identity. For example, do not say, "You are a trans* man, but you have a vagina, so that technically still makes me a lesbian." That may make your partner uncomfortable that you were even calling the area a "vagina." Remember how inconsiderate sexual identity labels can be when they require the assuming and labeling of others' bodies.
Do not call parts "real" or "not real" when using toys. That may cause dysphoria. Some who are trans* or queer may use toys to alleviate dysphoria. For example, by calling a strap on dildo "not a real penis," this may cause a trans* person to feel as though you do not legitimately believe it is a penis, and that you possibly may think other things about them are not legitimate either. Do not call things "real" or "not real," ever. After all, who is to say what is real? Who is to say what someone's gender is?
Conclusion
When it comes to dating trans* and queer people, and dealing with life in general, remember that you determine terms that apply to your own self, and others determine terms to apply to their own bodies. Negotiate, and learn.