Croup is diagnosed based on signs and symptoms. The first step is to make sure it is not another condition that can block the upper airway, especially epiglottitis (an inflammation of the tissue that covers the trachea or windpipe) , something in the airway, subglottic stenosis (narrowing of the airway below the vocal cords), angioedema (swelling underneath the skin), retropharyngeal abscess (pus in the back of the throat), and bacterial tracheitis (bacterial infection in the trachea).