If you consistently (over any given period of time) win then you can chalk it up to skill. If you predict stock moves consistently and gainfully then you can chalk it up to skill. If you can look at the past weeks or months and see a steady and progressive trend up in your account, you can chalk it up to skill.
On the flip side, if you are picking stocks at random, or with little thought and analysis, it is all luck, and you had better find another hobby soon.