Do a Cocker Spaniels ears as a puppy indicate how curly their ears will be as an adult?

If you get your pup from a responsible, reputable breeder he/she can show you both parents (the dam of course, but not all stud dogs are kept with the breeder) and tell you what to expect from each puppy. It all depends on genetics, and since puppy coats often change, you can't really determine what the adult will be like based on the puppy's ears. A good breeder will be able to help you select for what you want, or honestly tell you that none of the pups will have the straighter coat you desire. Contact your local breed club (Google "Cocker Spaniel breed club near --------------") and you can get in touch with people who love the breed. They can recommend good breeders to you, and more importantly what breeders to stay away from.

Genetics will determine the outcome, genetics can't be changed, however if you got it from a reputable breeder and viewed the sire/dam you should have a relatively good idea of how it will turn out or ask the breeder.

More importantly what you prefer in the type of coat is not that important, but the quality of breeding, which determines the dogs temperament, too much emphasis is put on appearances these days.

A well bred dog will conform to breed standard, regardless of your preferences in coat, should have done your research before getting any dog, especially if the coat is your only requirement, are you going to return it if it doesn't turn out the way you wish? Dogs are not something to be disposed of due to your superficial sense of vanity. That is not fair to the dog and it will suffer due to you possibly not wanting the dog, for such an insignificant reason, due to no fault of it's own.

Find yourself a reputable breeder, one who is breeding as close to the Breed Standard as possible. Many things change as a puppy matures, but the parents/grandparents would give you some clue as to how a puppy might turn out in adulthood.

Not necessarily they may change as they get older. If you ask to see the parents of the pups you will have good idea how they will turn out