To diagnose heartworm disease in dogs, veterinarians primarily use antigen tests that detect the presence of adult female heartworms. These tests are highly sensitive and are considered the standard for confirming heartworm infections. Additionally, microfilariae tests can detect the baby worms directly in the blood, though they are less common due to their lower sensitivity compared to antigen tests. Regular testing is crucial, as early detection plays a significant role in the effective management and treatment of heartworm disease.