1. One paragraph focuses on one subject and the next focuses on a different one that's related to the first one.
2. Sentences have structures like "____ is like this, while/but/however ____ is like that".
3. Sentences say something to the effect of "Both ____ are like this".
2 and 3 are also easy ways to identify compare and contrast essays if you simply apply this logic to the thesis statement in the introductory paragraph.