how do pet food manufacturers know their products are yummy?

They have taste testers, but they don't tell them that its pet food. Lol j/k probably feed it to a number of pets. And if they eat it than it must be good.

They use animals to taste test the food. But what tastes good to the pets also has to sort of appeal to the owners. I remember years ago reading about a pet food taste test which found(no surprise to many dog owners) that the food which smelled best to the dogs was the one that smelled like rotten fish. The report said that the animals not only liked eating it but "apparently enjoyed the product so much that they wanted to roll in it as well as eat it". Most pet owners would have a problem having food around the house that smelled like dead fish, so they modify pet food so that it appeals to the owners in some ways too.

No, my pup is for sure a quality control specialist. If it dont make it past the smell test, there is no way he is gonna try it. He eats human food as well so he knows what food is supposed to taste like. There are few canned or dry dog foods he will eat...

They don't - most pets will eat just about anything, and have a high propensity toward liver flavors. Check out all the different flavored pet foods and you'll find their ingredients are all very similar, regardless of "flavor."

People test them - there's nothing wrong with pet food that keeps humans from consuming it. I assume they test it with animals too.