How do I know if I am having identical or fraternal twins?

Depends. You can determine it based on ultrasound if there is only one placenta. However, often times you need to have babies tested after they are born. Even twins that "look" identical may not actually be identical.
Typically you can . Tell from an ultrasound, looking at the placenta or the sex of the babies, but sometimes you have to wait until they are born.