You could use the move Skill Swap, which swaps the opponents ability with yours temporarily.  This can be used to see if any of them have a hidden ability.
You could also see if one has a hidden ability in the beginning of the battle, such as when Cloud Nine activates in a Swablu horde from time to time. Use skill Swap then to figure out which one has the hidden ability. 
You can also look for signs of a hidden ability.  For example, Murkrow. It's hidden ability is Prankster. Keep your battle scene on and see which Murkrow is attacking. If a Murkrow uses a status move and goes before your Pokemon (given its faster), you'll know if one has prankster. 
Other than this, it's all up to chance whether or not you find a Pokemon with its Hidden ability. 
Hope I helped!