Diagnosis
To determine the presence of heartworms, your veterinarian will perform a blood test on your dog. The blood test will allow the doctor to determine the presence of an a product released by the female heartworm. This particular diagnosis is very specific to heartworms and tests are accurate more than 90% of the time. Sometimes, negative results can happen, due to a too low number of worms, early stage infections or no presence of female heartworms in the dog.
After your dog has been tested positive to this first blood test, you can also get your vet to test for the presence of microfilariae (larvae), using concentring filter test.
Finally X-ray can be done to evaluate how much lung damage has been caused by the heartworms.

X ray are performed to determine the amount of damages done to the lungs
Talk to your veterinarian today about heartworm medicine.