Basically if they already have a large membrane wall, they are in separate sacs
70% of the time that's frats. Boy/girl = frats 99% of the time
30% of the time they're di-di identical. If they are both boys or girls and look alike, you can do a DNA test after birth.
This is veeeery good news as you are avoiding some of the high risks that come with mo-di, mo-mo, and conjoined identical twins--they share placentas and more which means risk of twin to twin transfusion and extra special care
Big hugs and if you FB, search for Twins, Triplets, and more...multiples galore private group. Great support system, photos of twins. It's for parents and grandparents of twins and multiples only.