Puppy mills?

Ok so.... I have never really dealt with puppy mill operations. So I am wandering what are some of the common puppy mill signs? I would like to be educated on the subject so that it wont catch me off guard if I would find something of the sorts. Any advice on how to tell if a puppy mill org is being ran or promoted or trying to sell puppies would be great. As well as the typical signs as well. I would hate to not know about them and end up being caught off guard when purchasing a puppy in the future if I would run into a puppy mill org. Any info is greatly appreciated! Thank You!!!