It's not stupid at all. What you would do is install the site in a different location (such as test.yourdomain.com). Create a different mySQL database, username, password... and upload all of the files into that location. When you run the installer, call test.yourdomain.com (or www.yourdomain.com/test). Enter the new blank database info for it. For the license key, just enter your existing key plus "-TESTINSTALL" at the end.
Meaning if your key was 123456-789012345, you would enter 123456-789012345-TESTINSTALL as your key.
See the following guide for details.
Once it's installed, you have a new/vanilla IPB install. Just install the same 3rd party resources, themes, etc. You can use it to test things before doing it on your live site.