There is no exact definition to good fashion sense. What is fashionable to one may be out of the world for another.
Geographical location and the culture of an area have a huge role to play in defining the fashion trends of that particular area. You have some sense for fashion, if you are not dressed weirdly. Any man or woman is dressed fashionably if they look good, are comfortable, and are not frowned upon after wearing the same. One also needs to be trendy in order to be considered fashionable.
Fashion sense lies in wearing to please the on-looker and being comfortable in whatever you wear. In order to achieve this goal, you need to strike a balance between going with the majority and being different. Fashion sense can be inherited. The history of fashion is full of stories of sense and nonsense. Celebrities especially get into limelight for their sense and nonsense of fashion.