Cystic fibrosis patients who have pancreatic insufficiency have trouble gaining weight even with ravenous appetites, but with the help of pancreatic enzyme supplements, they can and do achieve a healthy, normal weight for their age and sex. CF patients who are pancreatic sufficient may actually be slightly overweight since their bodies work normally to absorb calories and nutrients from what they eat.
It’s impossible to tell if someone has cystic fibrosis just by looking at them. Everyone with CF has a different set of complications relating to their disease. Some have more lung problems, while others have more digestive issues. Some are barrel-chested because of the extent of lung disease. Some look just like their non-CF siblings or relatives.
The only way to truly know if a person has CF is if they tell you. Never assume.