In the directory where you plan to install Elgg, do the following:
RewriteEngine on RewriteRule ^testing.php$ modrewrite.php
This tells the webserver to load modrewrite.php when testing.php is requested.
In order to get this test to work on some virtual servers, you may need to add:
RewriteBase /prior to the RewriteRule line.
<?php echo "mod_rewrite works"; ?>
<?php echo "mod_rewrite does not work"; ?>
Now use your web browser to load testing.php. If you see "mod_rewrite works" your server has a working mod_rewrite instance. If you see anything else, including an internal server error, your server is not properly configured for mod rewrite.