Identical twins develop from the same fertilized egg and are essentially genetic copies of each other. Fraternal twins, on the other hand, develop from two separate fertilized eggs, and are no more genetically similar than any other set of siblings. Making this determination before the babies are born is not always easy.
A couple of findings on ultrasound can help. If a first trimester ultrasound between weeks 8 and 13 shows that the fetuses share one placenta(or are "monochorionic"), the fetuses are identical more than 99% of the time. However, if the fetuses have two separate placentas, they can still be identical. In this case, a second trimester ultra sound can help.
If the twins are different sexes (one boy, one girl), then they are fraternal. If they are the same sex (and have separate placentas), the determination can be made by genetic testing during pregnancy(either with amniocentesis or chorionic villus sampling) or by testing afterbirth.