Puppy mill or reputable breeder - how can you know?

We are thinking about getting a puppy at some point in the future – probably a few months. I need time to make sure this is what we really want and then do some research. I don’t want to rush into it.

I want a dog with no “baggage” so I am thinking rescues and shelters are out, but I have no idea how to go about getting a puppy from the right kind of place. I am perfectly happy with a mixed breed, say out of the paper, but I don’t know if that is the way to go either.