Nick22,
Hi! I'm really sorry that your fiance was raped. It's not easy to get over that kind of trauma and she will probably need help. Encourage her to talk to a professional counselor, if she hasn't already done so.
I'm a little confused and have a question. Why were you trying to conceive before marrying your fiance?
As far as determining paternity, your fiance should discuss the options with a health care provider. You don't want to put her or the baby at risk.
Blessings,
~ nmpb ~