It is better to diagnose social anxiety through observation rather than through administrating a test. Diagnosing a patient based on a test is not as accurate as observing a patient's behavior. A patient could give answers to a test which are not truthful in order to get a medication for recreational use. It is better to diagnose social anxiety through observation.
Tests are inaccurate, can be coached, or can just give false positives. Observations in social situations are typically more accurate. There are certain mannerisms that cannot easily be faked. Perspiration is a huge indicator of true anxiety. A person can fake other symptoms, but that one is the most common clue that someone is truly suffering from anxiety.
I think that being able to observe a person interact in day to day life would yield a much more accurate result then taking a test, I think social anxiety that is something that is obvious in some ways and subtle in others and a test may not catch that but observing someone would.
It is my opinion that social anxiety should be diagnosed through observation of the individuals experiences rather than a test. There are varying levels of anxiety, and these can differ from person to person and experience to experience. Social anxiety is not something that can be tested by a unchanging or standardized test.