How do you know if you're retaining water or putting on fat?

Two different things: If you do daily weights you can tell. Fluid retention can result in rapid weight gain over days, and you may have swelling in your lower extremities. If it persists, or you have other symptoms like shortness of breath, cough, chest pain, see your doc or go to the er. Some scales can help measure fat content in your body. Women gain fat around their thighs, men more truncal in their bellys.

Usually fat: Usually fat. Unless you have a rare form of adrenohypercorticolism, and some other rare endocrine disorders, and possibly heart failure, it's usually fat. If you're worried go and get checked out by your dr. Being fat is not a benign condition. Change weight no matter what the cause.

Water or fat?: People that are gaining weight in the form of fat have an increase of their abdominal girth and will need larger clothes. Their weight gain is also gradual, except when they are forming ascites. Patients that are gaining edema fluid can gain 1-3 lbs/day and have swollen ankles. They get up several times a night to pass urine. If you are forming ascites, or edema, see a physician for a treatment.