(i) To test the law of conservation of mass, a reaction would have to be carried out in which the mass of the reactants and the mass of the products are weighed and shown to be the same.
(ii) The law of definite proportions could be shown by demonstrating that no matter how a compound is obtained, the reactants remain at the same proportions by mass. This can be done by decomposing a compound and showing that the masses of the elements present are always in the same ratio.
(iii) To test the law of multiple proportions, two different compounds made up of the same elements would have to be decomposed. If the mass of one of the elements is kept constant the masses of other elements 'combining with that of the element in different samples would have to be in the small whole number ratio.