The strongest symptom of untreated CF is that the pancreas isn't able to produce pancreatic enzymes, thus the body can't digest fat or sugar. Untreated CF patients are usually really malnourished, small and really thin. Looks like anorexia even though the patients eat normal or large amounts of food.

So if you have a normal or above average BMI, you most likely don't have CF.

The reason why this is usually not listed as THE main symptom is that it's rather easily treatable with Kreon or similar medication.

Untreated CF patients die of malnutrition on average at age 4.

If you had life-long issues with malnutrition even though you eat enough, visit the closest CF ambulance near your place and do a sweat test. They are pretty simple and usually don't cost much (or anything at all), so there's no risk in doing one.