How. So you know if you have anxiety?

Some is normal. Everyone has some anxiety, i.e. Fear, about many areas of life; about relationships, our jobs, our health, our children... It would be abnormal not to worry about these things. It becomes a 'disorder' when it is so severe that it prevents function, or causes significant suffering. There are also specific anxiety disorders including phobias, obsessive/compulsive disorder, and panic disorder.
Anxiety. Anxiety is the bodies alarm of danger whether real or imagined. It is usually manisfested by fear, racing thoughts, a state of hyperalertness, elevated heart rate, abdominal and muscle cramps, increase pacing, insomnia, increase breathing (hyperventilation), anger with irritability. Many organ system can send this alarm out like your brain, hear, lungs or hormones. They calling out to be evaluated.