There will always be some who try to skirt the laws.
However, in theory, you should have paperwork showing origin of the dogs. Rescues generally get more adult dogs than puppies. You have a paper trail of where the dog came from. Either release paperwork from a shelter, or release of ownership signed by the owner surrendering the dog. And a copy of the adoption contract of where teh dog goes.
Puppy mills are not going to keep puppies until they are older - the longer they have to feed them, the more it cuts into their profit margin.
Also, at least in PA, dogs coming out of shelters and rescues have to be spayed or neutered. Puppy mills aren't going to spend the bucks to have every animal sterilized.
Finding a reputable rescue is just like finding a reputable breeder. The consumer needs to do their homework and look into anyone they are going to get a dog from. There are laws... there are people who do their best to circumvent the laws.... and people have some responsibility in looking out for themselves in any transaction.