Rapid weight gain over the last couple weeks. How do you tell if it's fluid?

Go see your doc... Rapid weight gain is best assessed by your doctor who could determine if this is because of retained fluid due to an endocrine (thyroid/adrenal glands), kidney or cardiac problem, or simply because of increased calories. Your doc would ask pertinent questions and perform a physical examination. Shortness of breath, swelling of your legs, feet, abdomen or around the eyes may suggest fluid retention.