Are you ever at the pond when the feeder goes off, Mark? If so, you won't have to guess whether the carp are eating the food - they're not subtle. If you see wallowing and splashing like a juvenile hippopatomus has found its way to your pond, those would be the GC happily enjoying the pellets you intended for bluegill. Although it could be CC, or a combination of both.
If you wear polarized sunglasses you should be able to see the fish eating the food, whether they be GC, CC, or bullhead.
