Richard,
If you mean "overrun" as the Actual Cost being greater than the Baseline
Cost then you can set up a custom field which provides a flag for any task
matching the criteria you've set up (i.e. Actual Cost > Baseline Cost). You
can also use the "Work Overbudget" filter to see only those tasks where Work
is greater than the Baseline Work. Whenever possible use the filters to
narrow down tasks to review.