If there isn't pesticide residue in the CAFO manure then there isn't pesticide residue in the meat produced at the CAFO that feeds conventionally produced grain.

In which case, what is the selling point of organic grain fed to livestock?

Is there any samples taken of the manure to verify/document no pesticides or is it simply good faith, sign a paper?

Think about the broadcast liquid manure application.     If there is no testing how would anyone know?