Step one. As you unpack chicks from the new shipment, dip each beak in water so they know to drink. If the chicks are active and moving freely, it's easy to spot a chick in trouble. You will see the chick standing still with its beak in the air, gaping it as you described. Or a chick will be standing still but wobbly, with eyes perhaps partially closed.
Step two. Have warm sugar water with a few drops of Poultry Nutri-drench ready to treat sick chicks. You can try to dip their beaks into the sugar water and try to get them to drink. If they won't drink, you need to syringe the water into the beak, going in on the chick's right side and just under the tongue to avoid the airway in the center of the throat.
Step three. Keep the entire shipment off the display floor until the next morning, retaining any chicks that are still not 100% for further chick aid.