Identical or fraternal?

Tiffany • This is my third pregnancy and we found out on October 14th that we are having fraternal twins due May 20th.
I saw my ob/gyn today and she did another US to check on the twins. She said she still can't tell if they are identical or fraternal. I will see another Dr that specializes in multiple pregnancies at 19 weeks. Has anyone else having twins gotten the same information? Is it normal to not know if the are identical or fraternal?