Intra-abdominal fat is a dangerous type of hidden fat that wraps itself around internal organs. It increases your risk of cardiovascular disease, high blood pressure, high blood cholesterol, diabetes and certain forms of cancer.
Short of getting a CT scan or a MRI, it is hard to know for sure. If the waist of your pants is getting tighter over time, and you are unable to pinch more fat, it probably reflects increasing intra-abdominal fat.
Just commit yourself to do a moderate activity (one that you enjoy!) on a regular basis. Swimming, for example, is a great cardiovascular exercise that also happens to be a great fat-burner. It is easy on the joints and unlikely to leave you with sore muscles or injuries