If she has her parent's names and date of birth they should be able to figure it out. Unfortunately, the bureaucratic process can be lengthy (filing paperwork, waiting the outcome, etc), when all in all it would take about 2 minutes to have an answer, at least I could if I had the information and official request. If she has her Alien Registration Number (ARN), or just her name and DOB, it will include the parent's names. If they have their names and date of birth, they can determine if they naturalized before she was 18. Filing paperwork with US CIS costs a lot of money and people very often do not file the derived citizenship of their children, although many times they take their naturalization paperwork and the whole family to get US passports because it is much cheaper and once you have a passport, you are forever a citizen, even when the passport expires. They have a form that is sort a flow sheet to determine citizenship, although I do not know where you get it, presumably at a passport office. Some problems that could arise are names that may have changed because of marriages or the person shortened their name and their last known name may not be the name they entered the US under. Good luck.