Economists, as well as courts dealing with antitrust cases, often use which concept to measure whether a firm has monopoly power in supplying a good or service in the given market?

They use crossprice elasticity to determine monopoly power. In this case, this is how responsive a change in demand is of one good to the change in price of related good. The more elastic a product is, the more likely the product is to change its demand when another good changes its price.