How can I find out if my php.ini/htaccess changes work OK?

Not all hosts allow to change PHP configuration values through php.ini files or .htaccess files. Sometimes only some items can be changed and others can't.

For example, your PHP memory limit is set to 8M, but you want to double that. In your .htaccess file, you have this line:
php_value memory_limit 16M

To make sure a change you made is actually making any difference, you can use Joomla!'s System Info:

• Joomla!: Help -> System Info -> PHP Information

Now search for 'memory_limit' on that page. In the 'Master Value' column on the right, you see the original value of 8M. In the left 'Local Value' column, you see the actual value. If this isn't your new value of 16M, then your host doesn't support overriding this value. You'll have to ask your host to change it.