This is difficult..
Without reading the specific essay, or even being able to research the essay or author (due to lack of information provided), it is difficult to answer this question.
However, a few general tips can likely assist you:
a) What is the topic of the essay? If the topic is approached in an obviously biased way, then the personal reason for writing the essay could be for persuasive purposes.
b) What is the thesis of the essay? If the thesis is persuasive in nature, then a personal bias and/or reason can be assumed. Keep in mind, however, that even a narrative or expository essay can be biased- those types are merely subtler than persuasive essays.
c) Textual evidence can be a summary of a portion of the essay, or it can be a direct quote. The best textual evidence is a combination of the two; in other words, the summary better explains the reason you chose a particular quote.
Hopefully this helps- if not, then please post a link or the title/author of the exact essay you need to analyze.