If anxiety is considered “normal,” when and how can we determine if someone is suffering from an anxiety disorder?

Anxiety is a normal emotion everyone may experience when faced with an imminent event that causes them to feel worried or uneasy, such as an upcoming job interview. However, although everyone may experience feeling mildly anxious from time to time, the difference between general anxiety and an anxiety disorder is distinguished by the cause of the anxiety, along with its duration and intensity.
Unlike general anxiety that comes and goes, those with an anxiety disorder typically feel worried or uneasy with no apparent or distinguishable cause. Characteristics include anxiety that is reoccurring, chronic, and generally more intense. Anxiety shifts from a normal emotion to a disorder once it begins to interfere with daily function.
Symptoms of an anxiety disorder often include physical changes, such as agitation, sweating, nausea, dizziness, insomnia, high blood pressure, and increased heart rate.

Let me be clear right from the start. Determining whether or not someone is suffering from an anxiety disorder should be left up to professionals trained in that field. Cases will differ from patient to patient, so I can only give a broad overview of the differences between general anxiety and what might be an anxiety disorder.
Feeling anxious is normal. Everybody experiences anxiety from time to time, and that doesn't mean a person is on his or her way to an anxiety disorder. A general key to feeling anxious is that it is triggered by an identifiable stressor. That stressor could be an upcoming exam, a recent fight with a friend, an upcoming job interview, or a new job. Once the stressor is removed, the anxiety goes away.
This contrasts sharply with a general anxiety disorder. Anxiety disorders will cause a person to feel anxious most of the time, and there isn't an identifiable stressor that is triggering the feelings of anxiety.
Differences also generally exist in the intensity and duration of the anxious feelings. For example, a person with a disorder will experience anxiety that is not proportional to the stressor. Worrying about exams months ahead of time is not typical of normal anxiety. Often, other symptoms also accompany anxiety disorders. Things like dizziness, sweating, heart pounding, headaches, and nausea are common. Frequently having to urinate is also a symptom.
https://www.mayoclinic.org/diseases-conditions/generalized-anxiety-disorder/symptoms-causes/syc-20360803

Anxiety is different from general and isolated worry and is often characterized by chronic and repeated feelings of tension and worry. This extreme worry can cause subsequent behavior, such as avoiding scenarios that may seem intimidating as a result of the anxiety. Anxiety can also manifest physically through symptoms such as high blood pressure, sweating, increased heart rate, insomnia, and dizziness.
One way to determine if someone has anxiety is to identify whether or not a traumatic event has recently happened or if an anniversary of a traumatic event has transpired. Traumatic events, and their anniversaries, can cause feelings of anxiety and depression.
Another way to determine if someone has anxiety is to identify whether or not their worries and dread have a clear cause. For example, if someone is afraid of needles, it would be expected that this person is made anxious when they have to get their annual flu shot. But anxiety disorder causes similar feelings without a clear, external justification.
https://www.britannica.com/science/anxiety