Which tool would a scientist use to find the mass of an object?

The common method would be to use Balance, If you think about it, on other planets, the balance weights change by the same factor as the object you are measuring. Your mass measured with a balance would be the same on the moon as it is on Earth. Weight is a measuring of gravity's effect on something. Mass is another story, it's the amount of matter in an object. Move to another planet and its object's weight will change, however, its mass will remain the same.