I do not know of a scale that does such a thing, but there is a test called a BMI. (Body Mass Index) This test basically tells the amount of fat you are carrying around. Some trainers at health clubs can do a BMI, but you can also ask your health care provider. (You should check with your doc before starting any exercise routine)
Hope that helps.
One Kg of muscle occupies a lesser space than an amount of one Kg of fat.
The more muscle you build the more fat you burn. Fat is stored energy in the body that comes from eating foods high in fat and high in carbohydrates.
If you do cardio regularly and lose 5 kgs of fat you will look leaner and thinner even if you put some muscle.
Body mass index can be easily calculated at home, just go to some online search engine and calculate your BMI.