An interesting and useful way to determine if the system clock has been set back is to sort Event Log records by event record number and observe the times...for each sequential record number, does the generated time for the record increment accordingly?
Another way to check for this (on XP) via the Event Log is to look for event ID 520, with a source of "Security". This event indicates the system time was successfully changed, and includes such information as the PID and name of the process responsible for the change, as well as the old system time (prior to the change) and the new time. An excellent resource and example of this is Lance's first practical.
Now, does event ID 520 necessarily mean that the user changed the system time? By itself, no, it doesn't. In fact, if you create a timeline using the image Lance provided in his first practical, incorporating the Event Logs, you'll see where event ID 520 is in close association with an event ID 35, with a source of W32Time...the system time was automatically updated by the W32Time service! You'll also find a number of instances where the system time was updated via other means.