This can sometimes be easily seen, especially in “polemical” essays, where the author bashes a number of points, truints or arguments and then presents her or his own. Or it could be more difficult to tell. Sometimes you have to “feel” it out, by assessing the tone or by watching for negative or positive adjective: “as so-and-so said in their excellent essay, ‘Nuke’ em Now!’” or “who shows a wrongheaded insistence.” Cues like those words can help you figure our where the author is coming from.