Tests are available that can help to identify infestation by the heartworm parasite.
Antibody tests are preferred for cats because they often have many fewer worms inside than dogs. A positive heartworm antibody test indicates that the animal most likely has an infection in progress (2-3 months at least).
The antigen test is more likely to have false-negatives, though false negatives can occur with the antibody test as well. However, if an antigen test is positive, it is usually a trusted result.
X-rays are of limited use in cats for definitive diagnosis since the infestations are usually low burden ones.
Ultrasound of the heart and lungs has a much greater diagnostic capacity. Ultrasonographers can often see the worms in the right ventricle chamber of the heart, or in the pulmonary artery.