Since I am still in the midst of lots of SharePoint work, one of the recent items I started on was a quick list of things to check when troubleshooting.  As SharePoint is a framework and a platform it's often hard to find out where issues are coming from, so I use this as a guide to track down errors:

• Check ULS logs with the ULS Viewer
• Verify web.configs for the affected Site
• Check the Event Log on the Central Administration machine or the Web Front End for errors
• Turn on/off Custom Error Reporting for Local/Developer/Test environments only
• Check IIS logs on the Web Front End for any errors not caught by ULS (this is a rare case)
• Clear page cache
• Assure the SharePoint User Permissions are correct

I also have a few locations I check to see if anyone else has my same issues

• MSDN Forums
• SharePoint Overflow (soon to be StackExchange Overflow)
• The SharePoint Homepage, to check for updates

I've found SharePoint to be difficult to check sometimes because its not just a web server, often pages are encapsulated within what are called WSP files, these are basically templates that pull data from a database and build the pages.  It makes tracking issues down complex as often the problem is code related, not a missing bracket or end tage on a page.