trogau

You can query core, plugin, and themes individually, or check all at once.
It defines four new methods:
<ul>
<li>xmlrpcCoreUpdateCheck – checks core, returns true/false</li>
<li>xmlrpcPluginUpdateCheck – checks plugins, returns true/false</li>
<li>xmlrpcThemeUpdateCheck – checks themes, returns true/false</li>
<li>xmlrpcUpdatesCheck – checks all of the above, returns an array of trues/falses.</li>
</ul>
The plugin was created primarily for WPUpCheck (http://trog.qgl.org/wpupcheck/), a simple 
Windows application that remotely polls a WordPress site to see if updates are available.

XML-RPC Update Check

This plugin provides XML-RPC methods for checking the update status of WordPress websites.

Gary Pendergast

Advanced Automatic Updates adds extra options to WordPress’ built-in Automatic Updates feature. On top of security updates, it also supports installing major releases, plugins, themes, or even regular SVN checkouts!
If you’re working on a WordPress Multisite install, it will properly restrict the options page to your Network Admin.
While this will be useful for the vast majority of sites, please exercise caution, particularly if you have any custom themes or plugins running on your site.

Advanced Automatic Updates

Nice Plugin

Adds extra options to WordPress' built-in Automatic Updates feature.

XML-RPC Update Check vs Advanced Automatic Updates