You can preview your pages at any time by either selecting Preview from the File menu or clicking on the Preview button 
in the Toolbar. In either case, you must first have specified a web browser to use in High Tea's Preferences. As some of the extensions supported here only appear to work in Netscape, you should be aware that document previews are browser-dependent and your document may look different in a different browser.
As whatever browser you choose gets loaded into memory alongside High Tea when previewing a document, your Mac will need enough available memory to run both High Tea and the browser (High Tea uses 1500 k by default). If the browser fails to open when previewing this is almost always due to a lack of memory rather than High Tea not workng (honest!).