Here's one topic I feel very strongly about. Whenever possible you should meet your breeder, the parents of the litter, the entire litter and even all of the dogs that your breeder owns.
Getting to Know Your Breeder
Exchanging emails and having phone conversations with your breeder is great. If you live a long distance away and are not able to go visit them than this might be your only way to get to know them. We imported Zephyr from Hungary and had to trust her breeder and his reputation when deciding to purchase her and even have her picked out for us! So, whether you can meet them or not, be sure you trust your breeder! Any one can get smooth enough to tell you what you want to hear.
If you are local to your breeder and they won't allow you to visit, hang up and find another breeder. Some may have reasonable restrictions--I wait until puppies are 10+ days old before allowing visits, but otherwise have an open door policy; this is not the same as not allowing visits.
What to Look For
The dogs - should be alert and friendly. This is not a people aggressive breed. Aggression is not the same as protection. Even dogs trained in protection are people friendly unless their owner/handler is threatened. If the dogs are people aggressive consider that this could be a part of their temperament they pass onto their puppies. If you can pet the dogs all over, feel safe in their presence, and they can interact with one another than you're probably in a good place!
How are they contained? Are they in kennels? Crates? Yards? In the house? Do the containment areas seem large enough for the dog? The minimum size crate for a mature bitch is a Large; I would not recommend anything smaller than a Giant crate for both bitches and dogs. Are their facilities clean of feces or debris? Turn around and get back in your car if the dogs are crowded into small kennels or crates that are dirty, offensive, or constructed in a make-shift manner.
How do they look? Are their eyes clear; coats shiny, weight good? Do they walk comfortable without limping or pain; if there is a pregnant bitch does she look healthy and seem mature enough for puppies? If everyone is big and friendly and smooth to the touch and clean than you're probably in a good place. If dogs seem under or over fed, are dirty, greasy or brittle feeling, or too young to be having puppies it's probably time to leave.
Look for puppies who are a good size for their age. They should be outgoing and brave; they should welcome being pet and handled. This shows you the breeder has spent a good amount of time socializing them and hopefully having multiple people interact with them. Puppies should have clear eyes, clean teeth, and soft coats. I would certainly prefer they have their tails as docking is barbaric and cruel. But, you may prefer docked tails. If the tails are docked make sure they are clean, not oozing and do not have an offensive smell.
The whelping box or kennel or room the puppies are in should be clean! There shouldn't be any more than the day's worth of puppy mess. The puppies should be reasonably clean - though, smearing food and other things on each other is a well known favorite puppy pastime! Momma should be with them or nearby. She should not look skinny or drained from her pregnancy even if she has 10 eight week old puppies! She should be clean, be at a reasonable weight, and look healthy. If she has been well cared for during her pregnancy and while nursing she will look like it.
Health clearances! A preliminary OFA is NOT an OFA. A preliminary is not even sent to OFA! It's an xray the breeder had taken and he or his vet looked at it and guessed what they thought the dog might receive as a rating. Some breeders will hope you are ignorant about OFA screening and call their dogs OFA'd preliminary. That could mean they had the xray taken, realized the dog was not going to pass a healthy rating and decided not to submit the xray to OFA but instead calls the dog Prelim and doesn't disclose the full truth. Make sure any dog old enough to be bred (never younger than 2 years) has at minimum OFA ratings for their hips or elbows. You can prove their results by looking them up on AKC or OFA. Ideally find a breeder who also certifies the Heart (very important in Rottweilers) and the eyes and thyroid. These can done through OFA and a few other organizations or by yearly echocardiogram, blood work, and eye exams by a qualified veterinarian.
What to Avoid
Avoid rows of dirty, small kennels full of aggressive dogs! Even friendly dogs may bark at the excitement of their human bringing a new visitor nearby. Barking alone isn't an indicator of aggression. Some of the best cared for dogs spend time in a kennel for short periods of time but do not necessarily live in the kennel 24/7. If the kennel is clean, provides an indoor space, has a way for dogs to elevate themselves off the floor and a separate space to eliminate than this is probably an okay place. When a breeder boards dogs, works with lots of intact males, has females in heat or pregnant who need to be separated, or does a lot of training it makes sense for them to have well constructed, sanitary kennels that are well protected from the elements.
Avoid any dog who can not be handled by the breeder or by you. Avoid any dog who can not be near other dogs. Not only could this be a sign of a temperament flaw that you do not want in your own puppy--it's also a sign that the breeder does not spend time socializing their dogs early and often. All of our dogs can be together: young, old, male or female. Again. be aware of times when dogs may not do well together such as more than one intact male or a bitch in season or pregnant. Use your best judgement.
Avoid any breeder who introduces you to the litter or even one of more of the parents somewhere away from their other dogs. Say, you found them online and they clearly have at least 5 or 6 dogs. Maybe more not listed. And you go to meet them and there is only mom and puppies? Time to leave. It is very common for backyard breeder types or for profit big puppy producers to only tell the public about their handful of very best dogs but to have several more that no one meets. They may show you puppies and possibly mom if she is still nursing at one location but have many many more somewhere else who they do not want you to meet because of any number of reasons - aggression, poor living conditions, an alarming number of dogs, etc.
Avoid any obvious signs of poor health in the puppies: dusty, brittle coats, runny or pale eyes, low energy, small size for their age, unsanitary living conditions, sneezing or coughing, diarrhea, being shy, timid, or unwilling to be handled, etc. Stay away from breeders who wean litters way too early -- in fact, if you can't meet mom with her puppies this is probably not a good place. Even after she stops nursing between 6-8 weeks (no sooner!) she still teaches her puppies valuable lessons about impulse control, bite inhibition, and socialization.
Finally, avoid breeders who seem to always have puppies available. If the breeder is specifically vague about how many litters they have there is a good chance it's because they have A LOT. They know this is not a good thing, that's why they hide it. No one person, even with a spouse and kids around to help, can possibly keep clean, socialize, and train several litters at one time. The more thinly their time is spread, the less attention your puppy will get. Stay away from any breeder who produces a large amount of puppies for sale cheaper than nearby breeders especially in an area where the breed is popular. This is someone looking to make a quick buck and cutting corners to do so.