What is a way to actually get a definitive diagnosis of copd?

PFTs... Pulmonary function tests will definitively diagnose COPD by demonstrating the presence of airway obstruction that is not reversible with bronchodilators. See your doctor to have this testing done!
Studies. To obtain a definitive diagnosis of COPD takes a combination of history, exam and test findings. Many times an xray can be suggestive of COPD and pulmonary function/breathing tests can be confirmatory.