Get a bathroom scale that measures %body fat as well as weight. As long as %Fat is going down or stays low, any added weight will be muscle or fluid.
How can a scale actually do that? The only way to actually measure %body fat, and even that method actually measure body density, is to be weighed underwater. This allows calculation of the volume of the body, which along with weight measure the regular way, in turn allows density to be calculated.
All a scale could do would be to use some approximation, probably body mass index (BMI), which tends to undervalue muscle mass. Using BMI, most NFL players, and even many NBA players, would be considered overweight and even obese. They aren't.