How to know if a topic is completely explored?
Welcome! The answer is usually that the topic is not completely explored. When you're new to an area, it can be hard to see what the open questions are. Hold off on asking the authors anything until you've asked your advisor and really dug into the topic (reading related papers, working through the problems). 

There's a nice answer to this question about approaching a new field:
Exploring something completely new, even silly.

Beyond that, I encourage you to look at the Illustrated Guide to the Ph.D.
http://matt.might.net/articles/phd-school-in-pictures/