I think a similar question already have been answered on this site. Nevertheless, let's try solving all your questions .
E-sports are really really new on the "job market" however, several people have been living as a pro level players for years. What does this mean? Since it has become an industry on its own, there will be more pro teams, more championships and therefore more jobs to sustain the demand.
After this brief introduction, let's dig into it. I am quite familiar with the selection process and what makes a person good candidate for the job (I know an e-sports team manager). The process is simple, after you submit your application, you'll be contacted and invited to a series of matches on the selected game. The performance is measured with multiple metrics: reaction time, team work, knowledge of the game (map, skills, timing, tactics). The actual win/lose isn't very important since that is just an output; is hard to build a relationship with people you don't know.
Finally, you will be evaluated to match certain duty: Slayer, support, lead, hybrid, tank... (This is the key to decide if you made it).
I'd like to add some personal thoughts. First of all, don't be discouraged if you don't get selected in your first try, with the increase of demand and crazy prizes (5 million dollars) teams will have extra players for backup. Secondly, during my game industry experience I learned that you can improve your playing skills if you invest time and think outside the box.
Hope this clarifies some points for you.