Hi Sanctus,
As a trans person myself, I think the most important thing when interacting with trans people is to be aware of the difference between biological gender markers and presentation.
In my opinion, if the person is presenting as female as clearly as the way you describe (make up, female clothes, feminine hair) then you need to assume that she wishes to be referred to with female pronouns and be respectful of that. Though it's unlikely a trans person would be offended by being asked which pronouns they prefer - being trans makes it necessary to be able to deal with situations like that - it might be be a bit of an emotional blow to someone who has gone to such an effort to present as a normal woman for someone to ask a question which so clearly comes from noticing the biologically masculine features they are trying to distance themselves from. She may find it particularly disappointing if she hopes that all her efforts help her to pass as a regular female. On the other hand, if you use "she" and "her" and "ma'am" without any prompting, you are likely to make that person very happy, as you are respecting the identity she is choosing to present to the world and boosting her confidence.
It is quite a different situation if the person presents androgynously. If you can't tell which gender someone is and they aren't offering any clear signals, or you're not sure if they are trying to present as a different gender than their biological sex, you should always ask which pronouns they prefer. This situation tends to happen a lot with female to male transpeople - we tend to go through an intermediary stage where you can't quite tell if the person is a masculine-looking woman or a feminine-looking man. I assure you this won't cause any offence. On the contrary, they will probably be extremely pleased that you are respectful enough of their feelings to ask the question.
In summary:
If the person is strongly presenting as a particular gender, regardless of whether you suspect they may have been a different sex biologically, you should use the pronouns of the gender they present as (they are giving you such a strong cue to their identity that it's not necessary to ask).
If someone is gender-ambiguous or you're not certain whether they are trying to present as one gender or the other, you should always ask.
Don't worry about getting it "wrong". People who are visibly trans have to deal with these things all the time, and won't mind at all if they have to correct you so long as you are being respectful!
I hope that helps!