Finding out the of the baby?

My sister in law went is pregnant and she went to the doctor to get an ultrasound last week. She was 17.5 weeks pregnant and her doctor didn't even try to look to see if ti was a boy or girl. Now her next appointment isn't until she is 20.5 weeks. Anticipation is killing us. Can't they usually tell at like 16 week? All of my family, who were pregnant, found out at 15-16 weeks. When did you find out?