As is the case before you purchase a dog, it's vital to evaluate the breeds before you rescue one. The American Kennel Club has a comprehensive list of registered Breed Rescue Clubs that can help you get started. Typically, a Breed Club will have a close relationship with their local corresponding rescue organization(s) for the breed they are devoted to. This is a sound resource to consult when you look for pros and cons of the dog breed you are thinking of adopting.
Reputable rescues will usually offer potential adopters a wealth of information about the dog breed in question. They are run by volunteers whose devotion to the breed and the welfare of the dogs they re-home is the reason they exist in the first place. Dedicated breed rescues know a lot more about their dogs than most people, so it's worth your time to read through their websites and/or contact them to discuss and ask questions. Rescue volunteers can be quite protective and go to great lengths to screen the adoption applicants because their goal is to find a forever home for each dog whose previous owners have failed them.
Don't be surprised or scared away by a rescue's strict evaluation of you and your home for adoption approval. Keep in mind that no one wants to help each of these dogs find the right home more than the volunteers! Their intentions have a lot of merit, and the best way to interpret their methods is that they are aiming for success, not just for the dogs but also for the adopters.
For example, if you have small children at home they will make sure you are matched with a dog whose prior history with children has been positive. A hyperactive dog might not be a good match for an elderly owner. A dog who's been known to chase cats will definitely not go to a home where the adopters have a cat.
Breed rescues use the criteria for placing dogs that, unfortunately, pet stores and irresponsible breeders do not.
Well you can never be completely certain. Not with a shelter dog nor with the most excellent pedigreed dog. Fears, aggression, dominance, and shyness are all emotional behaviors dogs can exhibit throughout their lives for a number of reasons. It can be due to past or present experiences with humans or other dogs, or it can even be due to medical reasons. Some will require more work and patience on behalf of the adopter, but knowing the possibilities and learning how to recognize signs of distress in a dog is vital to addressing the problem for everyone's benefit.
Here are some top-notch resources:
Learn from these videos and books how to evaluate the behavior and body language of shelter (or any) dogs. This will go a long way in selecting the right dog for you.