I have several Drupal sites and wish to have one site list all of their "status" and available updates. So to start with I am trying to find how to get the available updates.
The central site has access to the databases of all of the others so I am hoping to pull info direct from there but I have no clue where to look.
Is there a table with such information that is auto updated on cron etc?
You can use Drupal Remote Dashboard module for this
This module is for system administrators. It's a tool to monitor and manage any number of Drupal installations at once and it's built to be extensible by other modules. Since version 2.X this module also comes with the highest level of security where as all communication between the dashboard and each of the cores is AES encrypted, hence the dependency (see below). Optionally the monitoring data will be made visible in nice graphs if the Google Chart API is available.
Some of the child pages about Monitoring a site (which also has a childpage about Drupal Remote Dashboard, mentioned in another answer already) describe some other relevant topics, such as:
The table(s) these module operate on, are probably the table(s) you are looking for.
The contributed modules mentioned before typically offer canned reports / charts / queries. On top of that, you probably also want to consider all sorts of "Custom Report and/or Charts". For inspiration about what is possible in that area, consider looking at these questions / answers:
