Tricky question but how can someone distinguish anxiety from physical illness? how can you be sure that a symptom is purely psychological?

Anxiety: Anxiety is a combination of mental and physical symptoms. Often, mental stressors, for example like test anxiety, will worsen or produce physical symptoms like a faster heart rate, rapid breathing, sweaty palms. These physical symptoms can then magnify the mental symptoms and worsen the anxiety cycle. Treatment is to break out of the cycle on either the mental or physical sides.

Not so tricky: Sometimes it is difficult to make the distinction. But if you have a conscientious medical doctor who has run appropriate tests and ruled out likely medical illnesses that could cause your symptoms, it is makes far more sense to treat it as anxiety than to keep looking (often endlessly) for increasingly rare and unlikely medical causes.