Question

how will you test weather a given rod is made of iron or copper

Answer 1

By dipping it in Copper sulphate solution
If nothing happens , then the rod is of copper
If the solution turns green then the rod is of iron

Answer 2

You can drop both of them into water. measure the volume of water they replaced. now divide weight(actually mass) of both these materials by those volume of water they replaced. you will get the density. The material with more density is iron so the other one is copper.