A good medical work-up should help you with this. For example, if a person's peak flow meter is registering in the range of asthma attack, then it probably isn't anxiety. However sometimes anxiety has mimic physical symptoms. It's best to find out the parameters of the symptoms, for example, do they occur more often in the context of stress or not. Also, finding out more about what the patient is thinking at the time of the symptoms and what the symptoms mean to them (are they afraid of dying?) can be quite helpful in distinguishing. I'd be happy to talk more about it as I have 12 years experience in health psychology and run into these patients on a daily basis.