Twins with separate placentas aren't always fraternal. To know for sure, you need a twin zygosity DNA test.
Before DNA testing became common, doctors and parents believed that if twins had their own amniotic sacs and placentas, they must be fraternal.
Fraternal twins start when two eggs are fertilized by two different sperm. These fertilized eggs grow separately in their own 'in-utero homes' and can be girl/girl, boy/boy, or boy/girl.
Identical twins begin when one fertilized egg splits into two. This usually occurs around days 4 or 5, after the placenta and amniotic sac have already started forming. However, if the fertilized egg splits before the placenta and amniotic sac usually begin forming for identical twins, like around days 2 or 3, each zygote grows its own sac and placenta.
So, what might seem like fraternal twins to doctors is actually a pair of identical twins.