The time to ask if your established dog will get along with another dog is before you actually obtain the second pet.
While you cannot predict the behavior of a new puppy, you can try to learn your current dog’s tolerance level. This is done by introducing your established dog to a variety of other dogs.
If you already know the size of the dog you are planning to bring into the house, try to have meetings and play dates with other dogs of similar size and age (toy, small, medium, or large breeds and puppy or adult).
Where can you find a dog to help test this out?
Look to neighbors, friends, family members, and co-workers. Even some time spend at your local dog park may help give you an idea.
While the first one or two meetings should be outside on neutral territory, it is best to see how your established dog reacts to another dog while inside the home. This is because behavior can be radically different inside what a dog considered to be his territory.