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 *** (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.