Answer:
They can observe if the animal has a marsupium, or a pouch at the abdominal area, with a baby inside
Step-by-step explanation:
The marsupials have their name because they have a marsupium, or a pouch in the abdominal area and they carry their babies there, because when they born they are not fully formed and they finish their development there, and inside that marsupium are also the mamarial glands, so the baby can eat without going outside.