Identical twins are created from the same egg that has been fertilized and are basically DNA copy cats. Their genes are exactly the same. Fraternal twins are different because they are actually two separately fertilized eggs. They are the same as any other siblings but carried at the same time. However, knowing which kind of twins you are having before childbirth is not always easy.
One of the options available is to do an ultrasound early on to determine whether or not there are two placentas. If there are two separate placentas, then you are most likely to have fratneral twins on your hands. However, this is not always the case and another ultrasound should be done during the second trimester to be certain.
Another great way to tell the difference is to look at the babies’ genders. Two separate genders mean that the twins are fraternal. An amnio test is a possibility, and may work, but carries a small chance of miscarriage.