Baby too big

How do they tell if your baby is going to be too big to deliver vaginally ? I am going to be induced tonight and at my last appointment the doctor felt my belly and told me that my baby was definitely not going to be small but she didn't mention anything about finding out just how big he would be so I'm worried that I won't be able to deliver naturally. Do they do an ultrasound to find out when you go to be induced or what ?