l'd follow Celestial Hurricane's advice, l always use tactics like that to look for Pokemon with specific abilities, for example when l was hunting for an Inkay with Suction Cups and not Contrary, l'd use a debuffing move and see if it was reversed, or when Catching Abra for Synchronize, l'd use a status move.
Although regardless of what you do, you won't be able to tell which Pokemon you're affective in a Horde battle unless you kill them all off except one.