There is always a little corn in the manure when they are on a high grain ration.
If I'm looking at a stool and I see 4-8 kernels I probably won't get very concerned. But if I see 20-30 kernels I'm going to wonder why.
If the corn is cracked or rolled, you will have to wash the manure through a screen to see the corn. There is still corn in the manure, it's just harder to see because it is in small pieces.