What is the most effective method to test my writing skills?

There is no such thing as a standard measure to writing skills.  It's an art, and follows no pattern. Writers are made not born.  Meaning, you can develop writing if it's what you want to pursue.  And you can choose your own genre, too.  

If what you write and display can get at least a few attention or remarks and comments but positive if not either encouraging ones or even critical comments, then you can believe you are on it.