Twins!!! How do I know if they're identical?

Nicole • Married to the most amazing man on 9.28.14, had identical twin girls on 3.14.16, now expecting number 3 in May!
Just found out at the ER on Friday night that my husband and I are expecting twins. Yesterday we got a real ultrasound but the tech couldn't tell us if there was 1 or 2 sacs or placentas. It looks like they're super close so I'm leaning towards identical. But how do I know? She said I had to wait for my doctor to do an ultrasound but that's 10 weeks away