re: A trout in the milk
As I understand it, the reference is to the old sneaky practice of "watering" milk before you sell it, possibly by adding water from a nearby stream. The discovery of a small fish in your milk, while not direct evidence of anything other than a fish in your milk, would lead a reasonable person to deduce without direct evidence that the milk had been watered by the farmer.
Anybody can connect adjacent dots. The rest is a bit harder.