There is really only one way to know if you have social anxiety disorder, which is by going to your doctor and ask to receive an official diagnosis. Your doctor will probably refer you to a psychologist (or in some cases a psychiatrist) and they will be able to come to a proper diagnosis. Mental healthcare (just like physical healthcare of course) is so complex that you're not going to reach a proper diagnosis otherwise. A psychologist will usually ask you to fill in some questionnaires about how you feel in certain situations and have a couple of conversations with you. Based on that, they're normally able to give a diagnosis pretty fast, and a suggestion for a therapy to make your situation more managable, but they will only do things together with you and only if you agree with their suggestions. So you don't have to worry about suddenly receiving all kinds of therapies and treatments that you don't want, you'll be in control.
Anxiety is a feeling of unease, like fear or worry, that many if not all of all experience at some points in our lives. Most of us will feel worried about social situations occasionally. What makes it a social anxiety disorder is when you feel you have lost control of this and it begins to dominate many parts of your life. It becomes an intense fear that doesn't go away and affects your everyday life. Symptoms can be different for individuals, but if you notice that you have a long lasting, overwhelming and distressing fear of social situations, it is likely that you may suffer from a social anxiety disorder.