However if you are talking about water vs fat. Here's what I've found for you. It's nearly impossible to separate the two by measurement. Most people with years of experience can feel the difference in themselves. The easiest way to know is the speed at which the weight gain occurs. Fat take a while to accumulate. Water can happen in a day or two.
Your body (for a normal adult) is 70% water. So if you want to know how much of you is water... your weight - 70%. That answers that question. However, I think you want to know if you could weigh less if you lost water, right? That's not the answer. You Need Water.