How does drupal updates check work? I don't have an external crontab set up for my Drupal site.  How can Drupal search for updates in this case?
There should be a crontab task for Drupal. If there is none, it falls back to Poormanscron - integrated into Drupal core since 7.x


  The module inserts a small amount of JavaScript on each page of your site that when a certain amount of time has passed since the last cron run, calls an AJAX request to run the cron tasks. However, this approach requires that your site gets regular traffic/visitors in order to trigger the cron request.


When check is initiated, it loops over modules and checks their status from URLs determined in function _update_build_fetch_url define at line 287* of update.fetch.inc file, in update core module.

For more details, examine update core module, code is pretty well documented.




Information valid for Drupal 7.25, may slightly change with version.