Question

How to find the central idea when the author not state it explicitly?

Answer 1

Well you use context clues, (by the way this is called finding it indirectly :P), and visualize what the theme and message the author's tryna get across, like for example in The Lorax; They don't DIRECTLY state it, but based on the story sequence and structure, you could conclude that the central idea is to take care of Earth, and appreciate the things that comes out of it, because they may not be there for granted if we continue to pollute. c:

Answer 2

Most likely look for context clues
Hope this helps