Is it quiet personality or lack of social skills?

How can you tell the difference from lack of social skills and having a very quiet personality? My niece is very quiet and hardly ever hears him talk and when i talk to him he responses in short answers and i know for sure that he is not doing that to ignore me or anything because he does that to everyone else too. other people in the family says he just has a quiet personality buh im not sure which one it is. So how can you tell the difference from the two of them?