One difference between people who are shy and people with social anxiety, is that those with SA are very good at concealing it, whereas shy people have a more mechanical approach to dealing with social interactions. When you notice signs that are obvious, the person probably doesn't have SA, but is probably quite shy. I've met several of both. The shy people are much more difficult to meet, but they open up fast after you start talking with them. The socially anxious can sometimes seem quite rude at first, and they keep people at a distance for a long time before opening up to them. I find that those with SA tend to respond best to personal topics, believe it or not, and the shy can talk about anything. In both cases, their social skills tend to be quite good once you get past the shell. In fact, better than most. It's a completely different experience, meeting someone with SA and having SA.