Hello, my name is***** and I have over 20 years of experience as a veterinarian. I understand that you have chosen not to treat Dixie's heartworms and you wish to know how advanced her disease has become. We call this staging.
You won't be completely accurate just by observing her at home, but I can give you some guidelines.
Mild Heartworm Disease: showing no outward signs or a mild cough only. Heartworms are present but the body is compensating, mild inflammation present in the lungs.
Moderate Heartworm Disease: Cough, exercise intolerance (inability to work or play as intensely or as long as she once did), abnormal lung sounds. Heartworms are present, usually fair amount of inflammation and lung scarring, heart function no longer normal.
Severe Heartworm Disease: Cough, exercise intolerance, obvious difficulty breathing, abnormal heart and lung sounds, enlarged liver (hepatomegaly), syncope or "fainting spells" in layman's terms (temporary loss of consciousness from reduced blood flow/oxygenation of the brain), and fluid accumulation in the abdominal cavity, death. This stage is seen with severe inflammation and lung scarring, heart failure, organ failure (especially liver but possibly the kidneys too).
I don't think her barking has anything to do with her being heartworm positive, unless you mean a consistent barking cough, in which case she has progressed at least to moderate heartworm disease.