How Do I Know My Dog Has Heart Worm Disease?

Hello people who own computors and are hopefully nice enough to answer my question! lol. Anyway, I have a little 2-year-old black and white dog. It is a Shi-Tzu and POSSIABLY mixed with a Lasa-Apsa. She keeps making a honking sound when she is active. Is this typical for this breed? We thought she had Kennel Cough, and got her medication. It worked. Then, we ran out and now she is honking again. Could she have Heart Worms?! What are the symptoms and treatment for Heart Worms in dogs? Please, and Thank You!
I already took the dog to the vet. The test is expensive, and to much money. Does anybody know the symptoms of Heart Worm Disease in dogs?