Based on NZHawk's Awesome Update Finder, just type in the title id (If you don't know, it's the name of the game's folder) and this tool will show you all updates for that game along with relevant information.
How does this work? When you submit a gameid, my server downloads an XML from PSN (URL depends on the environment). Then, SimpleXML parses the document and splits it into a HTML (readable) format. In other words: magic. All information are public and are not obtained in any illegitimate way. The source to this page is available here and is free to use and redistribute under GNU General Public License 3.0. If you like this, consider donating.
Coded by Yifan Lu