I've been wondering about that, actually. How do you refer to a transperson if you're unsure of their preferred gender?
Use (singular) they. Many people in society don't know what zie and xir are, and its just easier to use they. Such as (in reference to an androgynous person) "They're just going out to lunch" or "They enjoy bowling."
Other than that, clothes are usually a pretty good tell. Not many transmen would be caught dead dressed in women's skinny jeans and a cami and I would only wear guy shirts if I like the design (such as band Tees). Make-up, hair style, shoes and other things which are usually divided amongst the genders are other good tells. However, if you're sure they're trans, asking is a good place to start. I wouldn't get offended if someone asked me about preferred pronouns nor would 90% of the trans people I know, but sadly I also know some very touch transpeople who would get overly preachy at you (through what I perceive would be no fault of your own). However, again make sure you know because you don't want to ask a masculine looking ciswoman if she's "transitioning" lest you enjoy being slapped.
But yeah, just use female pronouns for transwomen and male pronouns for transmen (the only exception I can think of is if they are presenting as their birth gender in front of a group, then resort to singular they so that you don't out them or cause them any dysphoria).
Dad called a transwoman 'it' in conversation because I dared bring up something in a Carl Sagan book about transsexuals being spoken of in a lot more politically correct light than I had expected.
So now my mood is in the tank.
Ugh, my mom once said in front of me after I began transition that "[Chaz Bono] is just a lesbian who is trying to look like a man."
Parents can really suck sometimes