If you plan to buy a dog of a certain breed, then you need to study the literature and specialized sites on the Internet to understand how to look like a puppy.
If you are unsure of your abilities, seek help from a professional breeder of the desired breed or kennel club. There are breeds (e.g. German shepherd), who at an early age is very difficult to distinguish from the usual stray dogs. These are unscrupulous sellers of animals. Not to get on their bait, you need to look at the puppy with a specialist who is knowledgeable in this matter.
In the case when the puppy got to your home accidentally, you can do the following: first, take newfound pet to the veterinary clinic so the doctors examined it for the presence of disease. It is possible that the doctor will tell you that "miracle" you brought off the street.
Another option is to include the puppy in the kennel club. There will also help to determine the breed of the dog.
You can try to look for information independently. On the Internet there are many sites with photos of almost all known breeds.
Often found a puppy can be a mestizo, that is, it has blood and pedigree dogs and mongrels. In this case, the experts are trying to determine what kind of "noble" blood flows in the veins of your pet.
Even if you bought or found the puppy was not a dwarf Spitz, and the usual "gorteria", do not worry, because the dog's love and devotion to his master does not depend on the natural belonging.