How do I print preview?

There is no print preview function. Pages is a WYSIWYG editor, so everything already aprears in it's final place and style.

You can use 'view' menu to show or hide invisible elements (like spaces and new line markers) and layout (borders around the text).

Also if you want to be sure how something will look like at the end without seeing page's interface, you can show a document preview (select file in finder window and press spacebar) or export to PDF.

You can preview your work in progress or final document in the following manner:

File > Print ... PDF > Open PDF in Preview