Adler provides two easy ways to test that you understand any proposition in your paragraph (not just the main idea):
1) Be able to state it in different words (your own words). The proposition is the thought the words represent, not necessarily the exact words themselves. That’s why different sentences can convey the same proposition. If you discover that you can’t state the proposition in your own words but can only repeat the author’s words, there is a high chance that you haven’t understood the author. You’ve received his words but not his thoughts.
2) Be able to relate the proposition to a personal experience you have had or that someone else you know has had. If that’s not possible, you should at least be able to provide an illustration or example of the proposition. If you cannot do any of these, there is a good chance you haven’t understood the author. You know you understand the author’s sentence when you understand each of the propositions in the sentence.
