Identical or Fraternal?

My doctor said its a little early for him to be able to see whether my babies are identical or fraternal, but what do you guys think? He said it was possible for identical twins to be in seperate sacs (that in fact it was preferable so they don't hurt each other), but it all depends on whether they're in the same gestational sac and share a placenta. This is my first ultrasound and first pregnancy ever, so I don't know much about what I'm looking at other than that there's two! What do you all think? This ultrasound was taken transvaginally at 7 weeks 3 days.