A Twin Zygosity DNA Test is a conclusive way to determine if twins are identical or fraternal. Identical twins come from the same egg and same sperm so their DNA is exactly the same and thus are their features. For identical twins, the DNA profiles are an exact match.
Knowing whether twins are identical or fraternal is often difficult. Appearance alone can be deceiving, since identical twins do not always look exactly the same, and fraternal twins often appear very similar. Twins sometimes grow up without knowing for sure whether they are identical or fraternal.
Identical twins often form in a single sac, sharing a placenta and amniotic membranes. An ultrasound during pregnancy can reveal whether twins share a placenta. If each has its own placenta, they are usually fraternal twins. But some identical twins will have their own placentas, and some will share, depending on when the fertilized egg split. So how it looks on an ultrasound is not definitive.