Also from that article, many of the famous and surviving cases fail to really have functional genitals at all (One of the oldest known survivors, Shiloh Pepin, was born without a vagina).
A DNA test? The child would either have XX or XY chromosomes.
EDIT: I should have read your post above mine, yes a DNA blood test would be the most likely option.
@Nightmare She died earlier this year, but was old enough at the time (early teens) to have a facebook page and such, where she humanized/popularized the disease for the general population. A documentary company picked up the story, and now there are fluther questions about it!