[Andreas]

error_log (at least this is the default filename) is apaches error logfile.
It's in the logs directory for apache.
But where this is on your system?
That's one thing you must know as it is your system

To check for mod_rewrite:

PHP Code:

if (functions_exists('apache_get_modules')) {
  if (in_array('mod_rewrite', apache_get_modules()) {
    // mod_rewrite is loaded
  }
} 


This does tell you if the module is loaded.
But this does not necessarily mean that it is being used.