Check if Drupal 7 module is enabled.

is_module_enabled

/**
 * param string $module
 *   Name of the module.
 *
 * return int
 *   1 or 0
 */
function is_module_enabled($module) {
  $result = db_query('SELECT s.status FROM system s WHERE s.name = :name AND s.type = :type', array(':name' => $module, ':type' => 'module'));
  $record = $result->fetchObject();

  return $record->status;
}