When it comes to buying, or adopting, a mixed breed dog most people do not care what breeds are in there, they just want a dog they think is cute and happy. Some people do want to know what kind of dog they have, and usually you can figure it out by their head shape, body shape, size and coloring. There is no guarantee that you will ever know exactly what breeds are in your mixed breed dog unless the people you got it from have both parents on site and know what they are.
Ask the owner, or whoever you are getting the dog from. Find out as much as you can about the dog your are adopting or purchasing. Even if you are getting it from a shelter they may have some idea of what kind of breed the dog is. They may also have some experts on hand that can help you figure it out.
Go to dog breeding or dog club websites online. You will want to compare the color of your dog, their body and head shape, possibly even its temperament.
Utilize the AKC's information. Going to the American Kennel Club's website could be helpful. Although it deals mainly with purebred dogs, the website still has information that can aid in picking out what kinds of breeds are in your one dog.
Ask your veterinarian. They can aide you in finding out what kind of dog your pooch really is. They have been trained to do this.
Get your dog's DNA tested. For around $80 you can search online for one of the many dog heritage places that do these tests. They will give you a list of primary, secondary and "in the mix" breeds in your dogs DNA. Most places only have maybe 100 breeds in their register so it is not 100-percent sure that you will find exactly what your dog is made up of.