How to determine if I have lost my baby teeth?

Good question. Some adults retain baby teeth. Children should have 20 baby teeth which exfoliate giving way to the adult teeth. Some baby teeth may not have adult teeth underneath them and may be retained. The only way to find this out is by going to a dentist and having either a panoramic x-ray or periapical x-ray.
Ask your dentist. Dental anatomy is a freshman year dental school course. All dentists can recognize the difference between baby and permanent teeth by visual examination and/or x-ray. There are differences in size, shape, position, color, and density of the enamel. Make an appointment with your dentist for an exam.