How to programmatically detect a CLI (Drush) context

Among other things, the $_SERVER and $_REQUEST is going to look totally different. Is that a start?
print_r() it and you'll see something you can grab.

The Drupal 7 solution to this would be drupal_is_cli(), which detects whether the current script is running in a command-line environment. (Nice.) Ex:

if (drupal_is_cli()) {
   return; // Good-bye Drush!
}

drupal_goto('http://www.google.com/');

To make sure that your code is being run from the cli and drush, you can check for the existence of drush functions.

Drupal 7:

if (drupal_is_cli() && function_exists('drush_main')) {
    // Code is being run from drush.
}

Drupal 6 (and probably 5):

if (PHP_SAPI == 'cli' && function_exists('drush_main')) {
    // Code is being run from drush.
}

Here is an info for D8 for completeness: https://www.drupal.org/node/2295037

D8:

// Just check the PHP_SAPI constant, directly
if (PHP_SAPI === 'cli') {
  // Do stuff
}

(saved you a click)