Time Machine - confirming when last real real backup was?

Time Machine makes one complete copy of your system, and then only copies over the files that have changed or been added in the last hour.

You can enter the Time Machine application and see what versions of a file are available.

In the Finder, you can navigate through the TM backup folders. You will find a folder date and timestamped for each snapshot (though bear in mind that snapshots are "consolidated" the older they get). This is only useful for showing you that a snapshot was made, not for showing what was copied, as this will appear to show you everything, not just the copied items. (TM uses links to reference unchanged files.)

There is also a perl script called TimeDog that can list the files copied over.
http://code.google.com/p/timedog/

There's also an app called TimeTracker, and a Widget called TimeMachine Buddy
http://www.apple.com/downloads/dashboard/status/timemachinebuddy.html

The simplest thing is to do a quick test. Create a file. Wait for a backup. Then delete the file. Then try to restore it from Time Machine.
This will give you an idea if TM is doing it's job. If you can't restore a file from your backup, then you have no backup.