You can preview what a Freeform HTML page will look like by clicking the "Preview" link to the right of any HTML file within Freeform Administration.

If you would like to test new code before putting it live, you can do this by just uploading a copy of the file with a different filename, like test.html. Then, click the "Preview" link underneath the Action column to see what it will look like. When you are happy with how it looks, just upload the file again with the original name, rename the test file, and/or make it a default file within a page.