Previewing a text means gathering as much information about the text as you can before you actually read it. You can ask yourself the following questions:
What is my Purpose for Reading?
If you are being asked to summarize a particular piece of writing, you will want to look for the thesis and main points. Are you being asked to respond to a piece? If so, you may want to be conscious of what you already know about the topic and how you arrived at that opinion.
What can the Title Tell Me About the Text?
Before you read, look at the title of the text. What clues does it give you about the piece of writing? It may reveal the author's stance, or make a claim the piece will try to support. Good writers usually try to make their titles do work to help readers make meaning of the text from the reader's first glance at it.
Who is the Author?
If you have heard the author's name before, what comes to your mind in terms of their reputation and/or stance on the issue you are reading about? Has the author written other things of which you are aware? How does the piece in front of you fit into to the author's body of work? What is the author's political position on the issue they are writing about? Are they liberal, conservative, or do you know anything about what prompted them to write in the first place?
How is the Text Structured?
Sometimes the structure of a piece can give you important clues to its meaning. Be sure to read all section headings carefully. Also, reading the opening sentences of paragraphs should give you a good idea of the main ideas contained in the piece.