It's tough to find out the boundaries when:
a) you can't really talk to her because she wants space
Wanting space is a boundary in itself Arashi, a very clear one, and one you should most definitely respect.
Arashi said:
b) you aren't sure if she will take offense to being "labeled"
You may talk with her and not mention PTSD at all, if you are concerned about the labeling aspect. Simply talk about whatever she is willing to discuss. Any talking is helpful and will build trust. Again though, only talk to her if she wishes; if she doesn't wish it, that again is her boundary; do not force the issue.
Arashi said:
c) the severity of the symptoms make them seem nonexistent most days
What do you mean by this? Severe symptoms are non existent? I am uncertain what you are trying to say here.
Arashi said:
d) she wears masks (like Anthony's post) to make everything seem alright almost all of the time
That is likely her way of coping, especially when faced with too many stressors. You cannot force her to be well, or talk to you, or anything else. As difficult as it is, part of loving someone with PTSD is learning to let go of them and let them heal at their own pace.
Remember Arashi, you cannot fix or change her, and you are not her therapist. It is commendable that you want to learn and do the best by her, however knowing what to do does not occur overnight. Oft times simply respecting their wishes and letting go is the best strategy. If you plan to be with her in the long term, PTSD will always be there, so you have plenty of time to learn about it; you needn't know everything immediately. It is a learning and growing process for you as well. Perhaps you need to examine within yourself, as to why you feel the need to help her so quickly. Looking inwards would be beneficial for you as well I believe. Take care.