One of my twins died in utero, will doctors be able to find out if they were identical or fraternal?

Doc should know: Likely your doctor has ultrasound images that can tell fairly well if the babies shared a placenta and/or sac or not. That's detectable as early as 9 weeks of pregnancy, assuming you had an ultrasound in the first trimester. If your loss occurred earlier than that in the first trimester, then likely the doctor cannot tell if they were identical or not.

Depends on timing: If the one twin died extremely early, there likely will be no way to tell. If you had an ultrasound, they might have been able to tell if they shared a sac or not. This might indicate the chances that they were identical vs. Fraternal. But the reality is, this is never going to be certain, and it is unlikely that you will have an answer. I am sorry for your loss!